Hi,
I’m running Pro Tools 10.3.7 (and have used 10.3.5) with the MR816 CSX (which I believe has the same drivers) and am not having any issues.
A couple things to try:
Have you uninstalled Pro Tools completely and reinstalled a fresh copy?
Have you tried changing the MR816 to a different firewire port?
I’m sure there’s a million different things that could be the issue, so try starting with the easiest and work your way up.