I also am having the same problem. Up until reading this forum, I have been opening my mail box before clicking the auction house NPC. My bank guy is in Undercity, so this has been rather easy. Did not connect it to opening my bags until now. The way it has worked pretty consistently for me is if I open wow and don't open my bag before clicking on the NPC Auction dude, complete crash.
I periodically get lua error boxes popping up on my screen, but I don't understand them. Some happen not related to the auction house and some on odd characters. I tend to just disable the add-on if I can figure out which one it is at the moment. If I can be of more help, I will be happy to reply. I just don't know what to do to give you the information you need.