As a first step, why don’t you try to trigger a rescan sudo -u www-data php occ files:scan --all

If that doesn’t improve things, try to find and delete the image file the log complains about misc/m-t0627-01511-00434 (2).jpg.

Still nothing after that, I’d try to hunt down individual contacts in the DB.

What happens if you delete this contact from the web UI?

Edit: Unclear whether the web UI is functional. If it isn’t, try deleting it from the database directly.

Using it to backup from a QNAP. Works very well and hassle-free. I’m using the QNAP backup app, but would be just as easy with any other tool. Just make sure to encrypt the backups.