Backup failed with error code 100031

I am using CB 2.2.127000.12 to back up simple copy for different folders (one at a time). The first time, the backup is successful for any folder. But the second time (after I make some changes in the source) fails immediately with error code 100031. What does this mean?

It might mean that a device is not properly functioning.
Please verify that both the source and the destination are ok.