Thanks for the quick reply @Shanling. I tried that yesterday before posting, but it didn't do anything.
As a follow-up, there was no change this morning. Red light is still on. M6 is still unresponsive. Force restart still doesn't do anything. Anything else you can think of that might help? We can discuss over p.m. if you'd prefer that.
This was issue that sometime showed on older firmware, but was always fixable by the forced shutdown/start on. It was supposed to be fixed with firmware 2.74.
Will continue troubleshooting with you over PM.