Just in case someone got the same problem but could not solve it.
I run NextCloudPi in a VM on Proxmox
So it’s a Debian Nextcloud setup.
I encountered the issue discribed above and got this page when navigating to my instance:
in my case this was due to a failed backup restore. I realised this as the used datadir space was smaller then the actual backup.tar.gz file.
After a renewed attempt, it worked fine.