I’ve rolled back to v5.5 for now. I’ve got 197 users who would are already grumpy with me for moving from Openfire to Mattermost 
Mostly due to notifications difference at the moment. I’m willing to try the upgrade again and help troubleshoot any way I can. By the way, I’m on the team edition if that helps. I did the upgrade from 5.5 to 5.6 skipping 5.5.1.