I installed OctoPrint 1.3.12 or later and now my MMU2 doesn't work properly/the whole printer resets, how to fix this?

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!

For further information take a look at this post and this issue on the bug tracker.