The "Error. Exception - sessionwaiterr" happens when your Moodle account locks up in the database. This can happen if it is processing large amounts of data or if too many sessions are opened in different windows and devices.

This error will clear on its own within about 15 minutes. You can also get around this error by either, clearing your browser cache or by using a different browser. This will cause a new session to be created that is not locked.