TYPO3 Core

TCA:

        'file' => [
            'exclude' => 1,
            'label' => 'LLL:EXT:filedownload/Resources/Private/Language/locallang_db.xlf:tx_filedownload_domain_model_file.file',
            'config' => [
                'type' => 'group',
                'internal_type' => 'file_reference',
                'allowed' => '*',
                'disallowed' => 'php',
                'size' => 1,
            ],
        ],

when opening the element browser in the backend the result is an exception:
#1288965219: $className must be a non empty string.

Reason: there is no element browser defined for file_reference only for file.
But: there is no special file_reference browser needed, the normal file browser would do.

The fix is very simple:
/typo3_src/typo3/sysext/recordlist/ext_tables.php
add this line after line 22:
$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['ElementBrowsers']['file_reference'] = \TYPO3\CMS\Recordlist\Browser\FileBrowser::class;

then the normal file browser would be called which works as expected.