OK, so I diagnosed it: this is what happens, which didn’t used to happen with Firewire computer.
With Thunderbolt, if you want to switch between a Firewire audio interface and Built-In Core Audio:
1. You must switch to Built-In input/output WHILE you are still plugged in with Thunderbolt (in my case, to RME 400). I run the Audiotester to make sure in/out are working.
2. Then you can disconnect Thunderbolt adapter (in my case, Firewire 800 adapter)
If you forget this, Max hangs (both Max5 and Max6) and will not launch.
If anyone has experienced something different please let me know, I appreciate!