The Unknown Error on a manual Debian setup usually means PHP or your web server is blocking the file transfer before it can even reach Xibo. Even if your final library folder has the correct permissions, you’ll want to check a couple of things:
First, check your php.ini file and make sure upload_max_filesize and post_max_size are set high enough for the media you’re uploading. Also, verify that PHP’s temporary folder (upload_tmp_dir) is writable by your web user.
If you’re running Apache, double check that LimitRequestBody isn’t cutting off the upload.
Honestly, chasing down these kinds of environment and permission tweaks is exactly why we highly recommend switching to a Docker installation if you can. Docker automatically configures all of these PHP limits and web server rules out of the box so you don’t have to manually troubleshoot the OS variables.