Backup failed with very old lock

[cdhowie]

Backup doesn’t take an exclusive lock anyway, so two backups can run concurrently. If backup is complaining about a lock then it’s an exclusive lock, which is only taken by some commands.

List of lock rules in documentation

I agree. Here is a table detailing which commands take which type of lock. Shared locks allow any number of other shared locks to exist at the same time, and these commands typically only read or add data. Only one exclusive lock can exist at any time, and it must be the only lock. These commands typically delete information from the repository, or otherwise require it to not change at all while they work. tl;dr: Shared allows other shared stuff to run. Exclusive doesn’t allow anything else …

I’m not sure why a lock would have suddenly appeared. Did you do any other maintenance on your repository within the last day, perhaps from a computer whose clock might not have been set correctly? Are you sure that the backups from previous days actually did run successfully?