diff --git a/typo3/sysext/backend/Classes/Controller/AbstractFormEngineAjaxController.php b/typo3/sysext/backend/Classes/Controller/AbstractFormEngineAjaxController.php
index e44810100e3d460f384029ef5c027f63847bc755..d15e4d600a8e1dc9c7e1ca0748dd66ed37d73f06 100644
--- a/typo3/sysext/backend/Classes/Controller/AbstractFormEngineAjaxController.php
+++ b/typo3/sysext/backend/Classes/Controller/AbstractFormEngineAjaxController.php
@@ -17,6 +17,7 @@ declare(strict_types=1);
 
 namespace TYPO3\CMS\Backend\Controller;
 
+use TYPO3\CMS\Core\Core\Environment;
 use TYPO3\CMS\Core\Localization\LanguageService;
 use TYPO3\CMS\Core\Localization\LanguageServiceFactory;
 use TYPO3\CMS\Core\Page\JavaScriptItems;
@@ -59,7 +60,8 @@ abstract class AbstractFormEngineAjaxController
     protected function getRelativePathToStylesheetFile(string $stylesheetFile): string
     {
         if (PathUtility::isExtensionPath($stylesheetFile)) {
-            $stylesheetFile = GeneralUtility::getFileAbsFileName($stylesheetFile);
+            // @see PageRenderer::getStreamlinedFileName()
+            $stylesheetFile = Environment::getPublicPath() . '/' . PathUtility::getPublicResourceWebPath($stylesheetFile, false);
             $stylesheetFile = PathUtility::getRelativePathTo($stylesheetFile) ?? '';
             $stylesheetFile = rtrim($stylesheetFile, '/');
         } else {