"Missing or invalid token"

After shutting down the PC yesterday, I was using my Android tablet for an on-line chat with some fellow conservation volunteers. After signing off, I tried to check iSpot, to see if an identification error I’d made (see “Conflicting genus” below) had attracted any more attention.
I was unable to log in (using Firefox as usual): the (saved, and unchanged,) username and password resulted in a “missing or invalid token” error message. Manually re-entering both fields was unsuccessful. The same data (also unchanged) work on the Android phone and - as I discover this morning - the PC.
Is this a known bug? And is there a “fix”?

Thanks to Mike D for the private reply. It’s likely more regular clearing of the cache will help.