The quick answer is - the OnStar report occurs once a month and it does not like ANYTHING (Progressive SnapShot, Range Module, DashHawk, DashBoss, etc.) plugged into the OBD2 port when it runs.
HOWEVER, the latest firmware (8.6+) has a more "stealthy" mode and is invisible, so it DOES NOT affect the OnStar report for most vehicles most of the time. HOWEVER, there may be a case where the Onstar hits the vehicle when running or driving, and at those times it MAY give an error.
The longer answer - OnStar runs diagnostics on the CAN bus at the same "address" as a diagnostic tool plugged into the OBD2 port (makes sense, right?) If something else is plugged into the port, it's like the address slot is already filled, so it can't run it's diagnostics. (Same thing happens with the dealer Tech II and any other device).
This doesn't hurt anything, but it will complain that it can't run diagnostics ... it usually occurs the same time every month and if you want, you can just update you module to the latest firmware and it should be transparent from then on.