OK, since we have verified that the quest is correct now. That's one possible error down. Can you attempt any other quest with handins to see if they work? This way we can verify it is the handin code itself and not something in the way it is called.
Sorry if this seems like a runaround, but I like to make sure what is the exact problem before I try to fix things. I have messed up enough quests by changing the wrong error before.
