This actually happened to ALL 5 rigs of mine and ALL 4 rigs of my friends that are also on bonuspool. It just kept spamming something like "invalid key" "JSON error" and i couldn't even bring up any menu options in CG. I thought CG was foolproof. Oh well, back to normal after close/reopen.
Also the first time it happened, was earlier this week and it just kept saying invalid share, rejected over and over and i had to close and reopen to get it to go to backup.
The error earlier the week could have been caused due to the slow getwork feeds, thats my best guess. Just let me know with as much info as you have at the time, whenever it happens again. Its faster for me to read the info via email if you could send it there next time around(although I dont expect that to occur again).