I had exact same issue:
And I was able to reproduce it on two Mac laptops (with Intel CPU) using Monterey 12.2 and 12.4.
Spend a day trying to figure out how to resolve this issue and since the only update I did was updating Unraid to 6.10.0 I decided to downgrade it back to 6.9.2. It actually worked and now my backups are running again with no issues