This can happen if you are updating your BBLog.
F.e. by the change from the old Battlelog layout to the new one. If this happens you have to delete the FULL folder from the Addon in the Browser addon directory.
Path under Win 7:
By both browser you have to search the right folder. The addon folder dont have the name of the addon, it have a mix from letters and number and special caracters.
Delete the detected folder (for security made a backup).
close your browser, wait a few secounds and open it again.
install BBLog completly new. now it should work.