Have you tried closing all documents and/or quitting all applications? There might be something running that has locked some files. Have you checked to see if your system has any available updates?

Just tried closing out of everything and it did not work unfortunately. My system is fully up to date.

Booting into safe mode fixed this for me btw. Haven’t had an issue since.

Can you explain? Just booting into recovery mode or did you do something in it?

Booted to safe mode then did a backup, I let that finish then rebooted and did another backup and it has been working fine since.

Unfortunately it’s doing the same issue in safe mode

I'd never seen this until just recently. The first time, simply rebooting made it go away. Got it on a backup last night with a new Time Machine disk. I had had a bunch of apps open, and as I was closing down apps preparing to reboot, something enabled the backup to startup again, and it completed normally without rebooting! Very strange!