@girish They are unusual I would say, yes. It's a file that is being updated on a daily basis so it has a file history of 30 different versions in NextCloud. They are .db.crypt12 files, which is a database backup of WhatsApp about 120MB in size.
@timmeh Your assessment is correct. If you switched the backups to rsync mode, you can remove the .tar.gz files. They are left overs from your previous backup. Maybe we can add an option to remove the old backup files (of previous configs). But of course, that will require cloudron to remember previous config as well.
Also, to be 100% clear, only remove the .tar.gz files. Do not remove any other files in the snapshot directory. The snapshot directory is the "working directory" of the backup system and stores the previous state.
Thank you @nebulon for solving this issue off-forums.
For future reference: problem was caused by the external storage being unmounted after a reboot. Remounting the storage drive (I use a netcup VPS) and then repairing in the Cloudron dashboard solved the problem. Including the mounting in /etc/fstab solved this issue for following updates/reboots.