@PHPMIKE, do you by chance have "Financial, Account, & Transaction" specified for the Detail Level of the Report Definition? If so, do you see different results exporting to Excel when choosing "Financial & Account" or "Financial" for the Detail Level?
The error/call stack you provided does look like a bug that we have already fixed in MR 2012 CU7, which has been released. If possible, you may want to try upgrading to CU7.