It slowly starts to become really frustrating: I didn’t want to annoy you longer with this individual problem, so I reinstalled the VPS.
Now I have completely reinstalled the VPS and installed discourse according to the official instructions. And after the initial configuration I was able to create the first small backup and actually download it by email. But after I have restored my last backup, the crap starts again: The download of the backup cannot be initiated again when clicking the link.
And it doesn’t matter which browser you use. I am registered in the browser in the forum where I also open the link in the mail in the browser.
The links look also similar to me so I guess something ist bricked in the forum configuration.
Backup-Link after first initial configuration:
https://my-forum.com/admin/backups/my-forum-2025-10-05-135900-v20250925182715.tar.gz?token=9***7468e4aa0da7f36aa6299872bc8d
Backup-Link after restoring my old backup and trying to create and save a new backup:
https://my-forum.com/admin/backups/my-forum-2025-10-05-141012-v20250925182715.tar.gz?token=9***7973b0d415528ee7308d3df8f637
the only way now is to save the backups at the vps directory directly. but that is really disappointing me, because I thought finally I found a forum software that is more or less foolproof.
if anyone has an idea what I could check, I’d be happy. starting the forum from scratch again is no option 
but at least you can backup and restore it the oldschool way as I figured out meanwhile 