I found this at the DUC when searching for your Assertion Error:
May be obvious, but make sure both Mountain Lion and PT are upgraded to the newest versions. Also check and see if your have to update your drivers to add support for Mountain Lion.
If none of that works, I'd search around the DUC.
Wish I could be of more help but I don't use the Mbox and my PT10 and Mountain Lion upgrade went off without a hitch so I didn't have to do a lot of troubleshooting.