There are a couple of things that seem to work well to get around this issue.  You could reboot the computer, close and reopen WSA, or sign out of Backup & Sync and sign back in.  Any of those actions seem to help in most cases.  If it's a recurring problem however, this is an issue best fielded through the support system since it's a case in which getting logs would be useful for gaining a deeper understanding of what's occurring on your system.

---