Unfortunately the problem still exists.
This time I tested it in a newer Xampp version (PHP 7.4.27) to make sure it's not related to my old version.
But the error is still there.
In your demo area, unfortunately, version 21.3 is still in use, so you can't verify the problem there.
The problem can be verified locally with Xampp but also on our rolled out Tikis with version 21.5 (PHP 7.3.xx).
Locally, the standard installation was clicked through and then elfinder was activated in the configuration wizard and also in the subsequent wizard window:
Set up File Gallery & Attachments: Only the first checkbox - Use elFinder UI
Set up File storage: Checkbox - Set elFinder as the default file gallery viewer.
If you then click on "Choose or upload image" in the toolbar in the editing window, it happens.