diff --git a/config/install/fillpdf.settings.yml b/config/install/fillpdf.settings.yml
index dce5b8d92663a1a37b8de426eff626c1a14ece2c..22e2228fd7db28c5d28f762307cb0f85dde5369a 100644
--- a/config/install/fillpdf.settings.yml
+++ b/config/install/fillpdf.settings.yml
@@ -2,5 +2,3 @@ remote_protocol: https
 
 # Should not contain a protocol. That's what the above is for.
 remote_endpoint: fillpdf.io/xmlrpc.php
-
-scheme: public
diff --git a/fillpdf.services.yml b/fillpdf.services.yml
index 091d7ab3859ee9c2c529369a3544a32e9c0b9dd8..591210bb2765d2f91625108ea17a302ffa0811d1 100644
--- a/fillpdf.services.yml
+++ b/fillpdf.services.yml
@@ -8,7 +8,7 @@ services:
 
   fillpdf.admin_form_helper:
     class: Drupal\fillpdf\Service\FillPdfAdminFormHelper
-    arguments: ['@module_handler', '@config.factory']
+    arguments: ['@module_handler', '@config.factory', '@stream_wrapper_manager', '@database']
 
   # I don't like the name of this, but it is what it does...it translates
   # the context provided by a FillPDF Link into loaded entities (or serializes)
diff --git a/src/Entity/FillPdfForm.php b/src/Entity/FillPdfForm.php
index 8db0819259ce6245cc6b578cc798c49a14109cba..9a30119bc894b20867d8f9e099474776dd85530f 100644
--- a/src/Entity/FillPdfForm.php
+++ b/src/Entity/FillPdfForm.php
@@ -120,15 +120,13 @@ class FillPdfForm extends ContentEntityBase implements FillPdfFormInterface {
       ->setLabel('Storage system for generated PDFs')
       ->setDescription(t('This setting is used as the storage/download method for generated PDFs. The use of public files is more efficient, but does not provide any access control. Changing this setting will require you to migrate associated files and data yourself and is not recommended after you have uploaded a template.'))
       ->setSettings([
-        'allowed_values' => FillPdfAdminFormHelper::schemeOptions(),
+        'allowed_values_function' => [get_called_class(), 'getSchemeAllowedValues'],
       ])
+      ->setDefaultValueCallback(get_called_class() . '::getSchemeDefaultValue')
       ->setRequired(TRUE)
       ->setDisplayOptions('form', [
         'type' => 'options_buttons',
         'weight' => 25,
-        'settings' => [
-          'options' => FillPdfAdminFormHelper::schemeOptions(),
-        ],
       ]);
 
     $fields['destination_redirect'] = BaseFieldDefinition::create('boolean')
@@ -154,4 +152,28 @@ class FillPdfForm extends ContentEntityBase implements FillPdfFormInterface {
     return $fields;
   }
 
+  /**
+   * Allowed values callback for 'scheme' base field definition.
+   *
+   * @see ::baseFieldDefinitions()
+   *
+   * @return array
+   *   Stream wrapper descriptions, keyed by scheme.
+   */
+  public static function getSchemeAllowedValues() {
+    return \Drupal::service('fillpdf.admin_form_helper')->schemeOptions();
+  }
+
+  /**
+   * Default value callback for 'scheme' base field definition.
+   *
+   * @see ::baseFieldDefinitions()
+   *
+   * @return string
+   *   The system's default scheme.
+   */
+  public static function getSchemeDefaultValue() {
+    return \Drupal::config('system.file')->get('default_scheme');
+  }
+
 }
diff --git a/src/Form/FillPdfSettingsForm.php b/src/Form/FillPdfSettingsForm.php
index 02f0f0bb93502554170f11bd95ecbf5515f6d5fd..88b56de95d456ffdacbd134f2fc3afd474294763 100644
--- a/src/Form/FillPdfSettingsForm.php
+++ b/src/Form/FillPdfSettingsForm.php
@@ -2,14 +2,20 @@
 
 namespace Drupal\fillpdf\Form;
 
+use Drupal\Core\Render\Markup;
+use Drupal\Component\Render\FormattableMarkup;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StreamWrapper\StreamWrapperInterface;
+use Drupal\Core\StreamWrapper\StreamWrapperManagerInterface;
 use Drupal\Core\Url;
 use Drupal\fillpdf\Component\Utility\FillPdf;
 use Drupal\fillpdf\Service\FillPdfAdminFormHelper;
 use GuzzleHttp\Client;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Link;
+use Drupal\fillpdf\Entity\FillPdfForm;
 
 /**
  * Configure FillPDF settings form.
@@ -72,13 +78,48 @@ class FillPdfSettingsForm extends ConfigFormBase {
 
     $config = $this->config('fillpdf.settings');
 
-    $scheme_options = FillPdfAdminFormHelper::schemeOptions();
+    $config_scheme = $config->get('scheme');
+    $schemes = $this->adminFormHelper->schemeOptions();
+
+    // Set an error if the previously configured scheme doesn't exist anymore.
+    if ($config_scheme && !array_key_exists($config_scheme, $schemes)) {
+      $error_message = $this->t('Your previously used file scheme %previous_scheme is no longer available on this Drupal site, see the %system_settings. Please reset your default to an existing file scheme.', [
+        '%previous_scheme' => $config_scheme .  '://',
+        '%system_settings' => Link::createFromRoute($this->t('File system settings'), 'system.file_system_settings')->toString(),
+      ]);
+
+      // @todo: It would be helpful if we could use EntityQuery instead, see
+      // https://www.drupal.org/project/fillpdf/issues/3043508.
+      $map = $this->adminFormHelper->getFormsByTemplateScheme($config_scheme);
+      if ($count = count($map)) {
+        $forms = FillPdfForm::loadMultiple(array_keys($map));
+        $items = [];
+        foreach ($map as $form_id => $file_uri) {
+          $fillpdf_form = $forms[$form_id];
+          $admin_title = current($fillpdf_form->get('admin_title')->getValue());
+          // @todo: We can simpify this once an admin_title is #required,
+          // see https://www.drupal.org/project/fillpdf/issues/3040776.
+          $link = Link::fromTextAndUrl($admin_title ?: "FillPDF form {$fillpdf_form->id()}", $fillpdf_form->toUrl());
+          $items[$form_id] = new FormattableMarkup("@fillpdf_form: {$file_uri}", ['@fillpdf_form' => $link->toString()]);
+        }
+        $list = [
+          '#theme' => 'item_list',
+          '#items' => $items,
+        ];
+        $error_message .= '<br />' . $this->t('Nevertheless, the following FillPDF forms will not work until their respective PDF templates have been moved to an existing file scheme:<br />@list', [
+          '@list' => \Drupal::service('renderer')->renderPlain($list),
+        ]);
+      }
+
+      $this->messenger()->addError(new FormattableMarkup($error_message, []));
+      $this->logger('fillpdf')->critical('File scheme %previous_scheme is no longer available.' . $count ? " $count FillPDF forms are defunct." : '');
+    }
 
     $form['scheme'] = [
       '#type' => 'radios',
       '#title' => $this->t('Template download method'),
-      '#default_value' => $config->get('scheme'),
-      '#options' => $scheme_options,
+      '#default_value' => array_key_exists($config_scheme, $schemes) ? $config_scheme : $this->config('system.file')->get('default_scheme'),
+      '#options' => $schemes,
       '#description' => $this->t('This setting is used as the download method for uploaded templates. The use of public files is more efficient, but does not provide any access control. Changing this setting will require you to migrate associated files and data yourself and is not recommended after you have uploaded a template.'),
     ];
 
diff --git a/src/InputHelper.php b/src/InputHelper.php
index a518281e3b3a164441338bd4766d10e267779bc8..d3931c08144a021d54178b298b3f5beed8aeb5be 100644
--- a/src/InputHelper.php
+++ b/src/InputHelper.php
@@ -43,7 +43,6 @@ class InputHelper implements InputHelperInterface {
       $fillpdf_form = FillPdfForm::create([
         'file' => $file,
         'title' => $file->filename,
-        'scheme' => $this->config('fillpdf.settings')->get('scheme'),
       ]);
     }
 
diff --git a/src/Service/FillPdfAdminFormHelper.php b/src/Service/FillPdfAdminFormHelper.php
index 15a3875bdd546b3c8e7e52f0b76a42cf3c45ef30..d7c49353731a8c36871cef09c78b8df62123c7a4 100644
--- a/src/Service/FillPdfAdminFormHelper.php
+++ b/src/Service/FillPdfAdminFormHelper.php
@@ -3,8 +3,12 @@
 namespace Drupal\fillpdf\Service;
 
 use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Database\Connection;
 use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\StreamWrapper\StreamWrapperInterface;
+use Drupal\Core\StreamWrapper\StreamWrapperManagerInterface;
 use Drupal\fillpdf\FillPdfAdminFormHelperInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 
 class FillPdfAdminFormHelper implements FillPdfAdminFormHelperInterface {
 
@@ -14,9 +18,25 @@ class FillPdfAdminFormHelper implements FillPdfAdminFormHelperInterface {
   /** @var \Drupal\Core\Config\ConfigFactoryInterface */
   protected $configFactory;
 
-  public function __construct(ModuleHandlerInterface $module_handler, ConfigFactoryInterface $config_factory) {
+  /**
+   * The stream wrapper manager.
+   *
+   * @var \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface
+   */
+  protected $streamWrapperManager;
+
+  /**
+   * The database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $connection;
+
+  public function __construct(ModuleHandlerInterface $module_handler, ConfigFactoryInterface $config_factory, StreamWrapperManagerInterface $stream_wrapper_manager, Connection $connection) {
     $this->moduleHandler = $module_handler;
     $this->configFactory = $config_factory;
+    $this->streamWrapperManager = $stream_wrapper_manager;
+    $this->connection = $connection;
   }
 
   /**
@@ -31,17 +51,46 @@ class FillPdfAdminFormHelper implements FillPdfAdminFormHelperInterface {
   }
 
   /**
-   * Returns acceptable file scheme options.
+   * Returns acceptable file scheme options for use with FAPI radio buttons.
    *
-   * Suitable for use with FAPI radio buttons.
+   * Any visible, writeable wrapper can potentially be used.
    *
    * @return array
+   *   Stream wrapper descriptions, keyed by scheme.
    */
-  public static function schemeOptions() {
-    return [
-      'private' => t('Private files'),
-      'public' => t('Public files'),
-    ];
+  public function schemeOptions() {
+    $site_default = $this->configFactory->get('system.file')->get('default_scheme');
+    $streams = $this->streamWrapperManager;
+
+    $options = [];
+    foreach (array_keys($streams->getWrappers(StreamWrapperInterface::WRITE_VISIBLE)) as $scheme) {
+      $name = $streams->getViaScheme($scheme)->getName();
+      $description = $streams->getViaScheme($scheme)->getDescription();
+      $string = '<strong>' . $name . '</strong>';
+      if ($scheme == $site_default) {
+        $string .= ' (' . new TranslatableMarkup('site default') . ')';
+      }
+      $string .= ': ' . $description;
+      $options[$scheme] = $string;
+    }
+
+    return $options;
+  }
+
+  /**
+   * Returns all FillPdfForms with template PDFs stored in a particular scheme.
+   *
+   * @return string $scheme
+   *   Scheme of the templates PDFs.
+   */
+  public function getFormsByTemplateScheme($scheme) {
+    $query = $this->connection->query("SELECT u.id AS form_id, f.uri AS file_uri
+      FROM {file_usage} u INNER JOIN {file_managed} f ON u.fid = f.fid
+      WHERE (type = :type) AND (uri LIKE :scheme)", [
+        ':type' => 'fillpdf_form',
+        ':scheme' => "{$scheme}://%"
+      ]);
+    return $query->fetchAllKeyed();
   }
 
   public static function getReplacementsDescription() {
diff --git a/tests/src/Functional/FillPdfSettingsFormTest.php b/tests/src/Functional/FillPdfSettingsFormTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..e6d10dd0e621d98fc2dd96e68c92981214259b79
--- /dev/null
+++ b/tests/src/Functional/FillPdfSettingsFormTest.php
@@ -0,0 +1,86 @@
+<?php
+
+namespace Drupal\Tests\fillpdf\Functional;
+
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\fillpdf\Traits\TestFillPdfTrait;
+
+/**
+ * @coversDefaultClass \Drupal\fillpdf\Form\FillPdfSettingsForm
+ * @group fillpdf
+ */
+class FillPdfSettingsFormTest extends BrowserTestBase {
+  public static $modules = ['fillpdf_test', 'file_test'];
+
+  use TestFillPdfTrait;
+
+  protected function setUp() {
+    parent::setUp();
+
+    $this->initializeUser();
+  }
+
+  /**
+   * Tests the settings form.
+   */
+  public function testSettingsForm() {
+    // FillPDF is not yet configured. Scheme should be initialized with the site
+    // default. Backend should be set to 'fillpdf_service'.
+    $this->drupalGet(Url::fromRoute('fillpdf.settings'));
+    $this->assertSession()->pageTextContains('Public files (site default)');
+    $this->assertSession()->checkboxChecked('edit-scheme-public');
+    $this->assertSession()->checkboxChecked('edit-backend-fillpdf-service');
+
+    // Now set the site default to 'private'.
+    $config = $this->container->get('config.factory')
+      ->getEditable('system.file')
+      ->set('default_scheme', 'private');
+    $config->save();
+
+    // Scheme should now be initialized with the new site default. Backend
+    // should still be 'fillpdf_service'.
+    $this->drupalGet(Url::fromRoute('fillpdf.settings'));
+    $this->assertSession()->pageTextContains('Private files (site default)');
+    $this->assertSession()->checkboxChecked('edit-scheme-private');
+    $this->assertSession()->checkboxChecked('edit-backend-fillpdf-service');
+
+    // Select 'dummy_remote://' as new default scheme.
+    $this->configureBackend('dummy_remote');
+
+    // Verify that the default 'dummy_remote' stream wrapper is present.
+    $this->drupalGet(Url::fromRoute('fillpdf.settings'));
+    $this->assertSession()->elementExists('css', '#edit-scheme-dummy-remote');
+
+    // Now uninstall the test module with the dummy stream wrappers.
+    $this->assertTrue(\Drupal::service('module_installer')->uninstall(['file_test']), "Module 'file_test' has been uninstalled.");
+    $this->assertFalse(\Drupal::moduleHandler()->moduleExists('file_test'), "Module 'file_test' is no longer present.");
+
+    // Reload the page and verify that 'dummy_remote' is gone.
+    $this->drupalGet(Url::fromRoute('fillpdf.settings'));
+    $this->assertSession()->elementNotExists('css', '#edit-scheme-dummy-remote');
+    $this->assertSession()->pageTextContains('Your previously used file scheme dummy_remote:// is no longer available');
+
+    // Select 'private://' as new default scheme.
+    // @todo Once https://www.drupal.org/project/fillpdf/issues/3040901 is in,
+    // this should be set via the UI:
+    // $this->drupalPostForm(NULL, ['scheme' => 'private'], 'Save configuration');
+    // $this->assertSession()->pageTextNotContains('An illegal choice has been detected.');
+    $this->configureBackend('private');
+
+    // Now remove the private path and rebuild the container.
+    $settings['settings']['file_private_path'] = (object) [
+      'value' => '',
+      'required' => TRUE,
+    ];
+    $this->writeSettings($settings);
+    $this->rebuildContainer();
+
+    // Reload the page and and verify that 'private' is gone, too.
+    $this->drupalGet(Url::fromRoute('fillpdf.settings'));
+    $this->assertSession()->elementNotExists('css', '#edit-scheme-private');
+    $this->assertSession()->pageTextContains('Your previously used file scheme private:// is no longer available');
+
+  }
+
+}
diff --git a/tests/src/Traits/TestFillPdfTrait.php b/tests/src/Traits/TestFillPdfTrait.php
index c2356e7cef7790e85c7b7fb5749852e70394c20c..874f4a05b86fcf03418b89d74a5fa260b069b2b3 100644
--- a/tests/src/Traits/TestFillPdfTrait.php
+++ b/tests/src/Traits/TestFillPdfTrait.php
@@ -11,14 +11,20 @@ trait TestFillPdfTrait {
 
   /**
    * Configures the FillPdf test backend.
+   *
+   * @param string $scheme
+   *   (optional) The file system scheme to use for PDF templates. Defaults
+   *   to 'public'.
+   * @param string $backend
+   *   (optional) The backend to use. Defaults to 'test'.
    */
-  protected function configureBackend() {
+  protected function configureBackend($scheme = 'public', $backend = 'test') {
     // FillPDF needs to be configured.
     /** @var \Drupal\Core\Config\Config $fillpdf_settings */
     $fillpdf_settings = $this->container->get('config.factory')
       ->getEditable('fillpdf.settings')
-      ->set('scheme', 'public')
-      ->set('backend', 'test');
+      ->set('scheme', $scheme)
+      ->set('backend', $backend);
     $fillpdf_settings->save();
   }