Default GL Account on PO Releases

I've just helped a client upgrade from 9.04 to 9.05.700, and we're experiencing a problem with creating a BTO purchase order. When we click on the "Get Default GL" button on the release, it's throwing up an error that says "The Account cannot be blank".

Interestingly, we tested this process out fairly thoroughly, and is I enter the sales order and this BTO purchase order on the last copy of the database that we converted as a test to the go-live, I don't have that problem.

To check this, I've turned off all BPM's and customizations, still get the error. I've looked at all the GL Control file setups that I can think of between our test system (where it works) and our LIVE system (where it doesn't), and everything appears to match up.

Has anyone experienced this problem before? Alternatively, does anyone have a nice document that describes everywhere Epicor goes to determine the GL Control account it uses?

Thanks in advance,

Kevin Simon