I was ready celebrate that my upgrade to macOS Vista Catalina was successful. Zero issues, zero data loss, only the expected barrage of security theater dialogs. And then Time Machine backups started to fail.
I have been using Time Machine with a Synology NAS to backup my mac for multiple macOS releases. I switched to this setup from an AirPort Time Capsule (RIP) years ago. I have never had any issues until now, and I have not changed any settings.
Before upgrading to Catalina, I followed Howard Oakley’s excellent recommendations. I made one last backup on Mojave. I completely disabled Time Machine. I saved and archived my old Mojave backups. After upgrading to Catalina, I re-enabled Time Machine and started a fresh backup set. Everything worked for a few weeks.
Now, my backups have started failing. Sometimes it cannot find the disk, even when it is mounted in Finder. More recently, it says that my .backupbundle “could not be accessed (error 45)”.
This Apple Community forum post from 2009 mentions “error 45” and users who replied said that Time Machine is not compatible with a NAS. That may have been true at the time, but it certainly is not true now. This Apple Support document from 2019 specifically mentions NAS support and links to the spec on Time Machine over SMB. I know my NAS is compatible and it was working perfectly prior to Catalina.
So far, the only solution I have found is to disable Time Machine backups, remove the NAS as a backup disk, re-enable Time Machine, re-add the NAS as a backup, and select the option to keep the existing backups. After this backups will succeed for a few days, or maybe a week. Then they start failing again.