No I have not encountered the same problem on your server (which is running much more smoothly than our university one).
I was just wondering if someone had reported something similar and if and you or one of the forum's participants might have run into the same type of difficulty.
I suppose the error comes when a user gets a grade from a task and TC does not know to what chain it belongs (that just me trying to make out from the meaning of the message )
Could it be a Database error?
Any thought appreciated.