Bad file ID? Just to close this one out, I contacted MSP360 support and, after reviewing my backup logs, they suggested the following:
It seems that the sync between Cloud storage and Local database is slightly outdated.
Please synchronize the repository following these steps:
1. Go to Settings - Application - Synchronize Repository.
2. Choose your account from drop-down menu.
3. Press Synchronize Now — MSP360 support
I did that and am happy to report it seems to have solved the problem. How my cloud storage and local database got out of sync in the first place remains a mystery. At least now I know what to try first if it happens again.
And kudos to Anton at MSP360 for the fast reply and resolution.
Brad