I would suggest moving the quest giver Heyda just outside of the small room that allows entrance to the cellar, i have seen this a few times now where someone has retreated out of the quest area and Heyda comes to assist and then blocks access to the cellar for the next person who takes the quest.
http://www.escapefromundeath.com/forums/showthread.php?t=62490
Ah didn't see that :P Well its not really a bug but i guess it works here as a bump.
The easiest way around it actually might be to make the exit transition a placeable so bats can't follow you through, thus eliminating the problem.