I was just reviewing this error to try and determine the issue with this error and I find two returns that had processed through MD without error with Activity Worksheets.  I have both Schedule E activity and Schedule C activity both with errors and accepted depending on the client.  Additionally, the clients that have approved and failed have both losses and profits in those activities.  So far in my review of the failed MD return clients, I don't see a consistency that would cause them error while the others have approved.