TYPO3 Core

In the log I can see the following error message:

Core: Error handler (BE): PHP Warning: Undefined array key 1 in /var/www/typo3/vendor/typo3/cms-filelist/Classes/ElementBrowser/FileBrowser.php line 63

It points to:

$disallowed = str_replace('disallowed=', '', $fileExtensions[1]);

The URL of the file browser contains the value 'disallowed':

wizard/record/browse?token=88681af69375cd93bd3a7f5d8767d480801abb90&mode=file&bparams=|||allowed=gif,jpg,jpeg,pdf,png,svg,tif,tiff,webp;disallowed=|data-64-tt_content-3-image-sys_file_reference

--> The error message seems to have been fixed with #101231, but it is still not possible for me to transfer files from the file browser to the content element.

Attached is a video to clarify. In file browser neither clicking on the file name nor on the + button causes the file to be applied. After selection via checkbox, the loading bar loads very slowly and the file is still not applied. In browser console no error message is shown.

As an addition (if relevant):
The feature toggles for CSP were initially activated in the project, but this has now been deactivated.

'SYS' => [
    'features' => [
        'security.backend.enforceContentSecurityPolicy' => false,
        'security.frontend.enforceContentSecurityPolicy' => false,
    ],
],

Patch set 1 for branch 12.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/80170

I can confirm, that it works with 12.4.2 and the issue shown in the video is present in 12.4.3 and 12.4.4
It seems to be triggered by this change https://github.com/typo3/typo3/commit/4c40d747e4

If anyone stumbles across this issue at a later time: more insights might be found in the slack thread regarding this topic
https://typo3.slack.com/archives/C03AM9R17/p1690209557387129

Applied in changeset a36132726cdee1d76a22aeddffd811195d3efb98.