Time machine backup failing but "verify backup" successful

I use Time Machine on a Synology NAS running DSM 7.2, using SMB2 or higher.

Time Machine backup worked well for 8 months and suddenly started failing with the error "The network disk disconnected from your Mac while backing up".

"Verify backup" is completing fine, that's the strange part.

When backing up, it does backup some data, but at some point it fails with the above mentioned error.

What should I filter for in the console? "mds"?