Some people have run into weird reset and loop issues with their MMU2 and OctoPrint 1.3.12. The cause of this - as far as the currently available information suggests - are simply misconfigured printer profiles inside OctoPrint.
If you have a multi-extruder setup like the MMU2, make sure your printer profile has configured all available extruders . In the case of the MMU2, you should have five extruders and a shared nozzle configured in OctoPrint. For an MMU it's four extruders and a shared nozzle.
Failure to properly setup your printer profile will cause issues!