I could now reproduce your problem. For this it is required to disable the view 'Images' option for the 'Registered' Usergroup in the jDownloads User Group Settings. See pic.
It is a simple bug and affects all PHP versions, not only PHP 8.
Attached a fixed version.
Please copy the unzipped file to this folder: /administrator/components/com_jdownloads/tables/ and overwrite the older file. Please test then again to create a new Download in the frontend as registered user and post here your result.
Thanks a lot.