Don't you think there could be a problem with the region counter ?
From your description and screenshots, I'm afraid I cannot see how this would be related to the region counter. Also, as til had mentioned previously, embOS RX IAR is used in a lot of products without any issues. Still, we'd of course ike to be able to reproduce the observed problem for a better understanding of what actually happens with your application. A screenshot of the call stack once inside OS_Error() could help with that. A copy of your SystemView recording could help with it as well. Ideally, you could provide us with a project for reproduction at our facilities, but I guess that still is no option. Or is it?