diff --git a/typo3/sysext/core/Classes/Resource/ResourceCompressor.php b/typo3/sysext/core/Classes/Resource/ResourceCompressor.php
index 6c3f4999bb3420874c72af87bc640b46df75bb01..6ab6d889d523d2b4df4d4697653026310beb38db 100644
--- a/typo3/sysext/core/Classes/Resource/ResourceCompressor.php
+++ b/typo3/sysext/core/Classes/Resource/ResourceCompressor.php
@@ -432,11 +432,12 @@ class ResourceCompressor
 
         // if the file is an absolute reference within the docRoot
         $absolutePath = $docRoot . '/' . $fileNameWithoutSlash;
-        // if it is already an absolute path to the file
-        if (PathUtility::isAbsolutePath($filename)) {
+        // If the $filename stems from a call to PathUtility::getAbsoluteWebPath() it has a leading slash,
+        // hence isAbsolutePath() results in true, which is obviously wrong. Check file existence to be sure.
+        // Calling is_file without @ for a path starting with '../' causes a PHP Warning when using open_basedir restriction
+        if (PathUtility::isAbsolutePath($filename) && @is_file($filename)) {
             $absolutePath = $filename;
         }
-        // Calling is_file without @ for a path starting with '../' causes a PHP Warning when using open_basedir restriction
         if (@is_file($absolutePath)) {
             if (strpos($absolutePath, $this->rootPath) === 0) {
                 // the path is within the current root path, simply strip rootPath off
diff --git a/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest.php b/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest.php
index 6a0190048362bf967708b93c68c0821b1d22dcd9..c01583e284a8a6cb0b033a44448894b4c17475df 100644
--- a/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest.php
+++ b/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest.php
@@ -558,13 +558,23 @@ class ResourceCompressorTest extends BaseTestCase
                 'typo3temp/assets/compressed/.htaccess',
                 '../typo3temp/assets/compressed/.htaccess'
             ],
+            // Get filename using absolute path
+            [
+                Environment::getPublicPath() . '/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css',
+                'sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css'
+            ],
+            // Get filename using docroot relative path
+            [
+                '/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css',
+                'sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css'
+            ],
         ];
     }
 
     /**
      * @test
      * @dataProvider getFilenamesFromMainDirInBackendContextDataProvider
-     * @param string $filename input that will be fired on the extension
+     * @param string $filename
      * @param string $expected
      */
     public function getFilenamesFromMainDirInBackendContext(string $filename, string $expected)
@@ -590,6 +600,57 @@ class ResourceCompressorTest extends BaseTestCase
         self::assertSame($expected, $relativeToRootPath);
     }
 
+    /**
+     * @return array
+     */
+    public function getFilenamesFromMainDirInBackendContextInSubfolderDataProvider(): array
+    {
+        $subfolderFake = basename(Environment::getPublicPath());
+        return [
+            // Get filename using absolute path
+            [
+                Environment::getPublicPath() . '/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css',
+                'sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css'
+            ],
+            // Get filename using docroot relative path
+            [
+                '/' . $subfolderFake . '/typo3/sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css',
+                'sysext/core/Tests/Unit/Resource/ResourceCompressorTest/Fixtures/charset.css'
+            ],
+        ];
+    }
+
+    /**
+     * @test
+     * @dataProvider getFilenamesFromMainDirInBackendContextInSubfolderDataProvider
+     * @param string $filename
+     * @param string $expected
+     */
+    public function getFilenamesFromMainDirInBackendContextWithSubFolder(string $filename, string $expected): void
+    {
+        // getCurrentScript() called by PathUtility::getRelativePathTo() is usually something
+        // like '.../bin/phpunit' in testing context, but we want .../typo3/index.php as entry
+        // script point here to fake the backend call.
+        $bePath = Environment::getBackendPath();
+        $subfolderFake = basename(Environment::getPublicPath());
+        $_SERVER['ORIG_SCRIPT_NAME'] = '/' . $subfolderFake . '/typo3/index.php';
+        Environment::initialize(
+            Environment::getContext(),
+            true,
+            false,
+            Environment::getProjectPath(),
+            Environment::getPublicPath(),
+            Environment::getVarPath(),
+            Environment::getConfigPath(),
+            $bePath . '/index.php',
+            Environment::isWindows() ? 'WINDOWS' : 'UNIX'
+        );
+        $subject = $this->getAccessibleMock(ResourceCompressor::class, ['dummy']);
+        $subject->setRootPath($bePath . '/');
+        $relativeToRootPath = $subject->_call('getFilenameFromMainDir', $filename);
+        self::assertSame($expected, $relativeToRootPath);
+    }
+
     /**
      * @return array
      */