We've done this a few times and the error is still occurring. Note that the purchases are going through on the QBMS side, and if you refresh the page after getting the error message, you see the confirmation information. The only piece of information missing is an "Invoice Number" and we have a very slight suspicion that the error is happening due to that piece of info, but could be a red herring.

I didn't see this post until after my last reply. I am almost competely certain that this is a bug in the store module and not due to an error in configuration. I contacted the developer to help me solve my issue, and it seems to me that that would be your best course of action. It is possible that the code that fixed this problem for me is not included in what you have, and no amount of fiddling on your end will change that, unless you have the source that is. They were able to get me updated and running fairly quickly, and have been very helpful.