I have encountered a bug a couple of times where UM freezes.
Because AutoDJ with Waitlist and crossfade does not operate as needed, I use pause between tracks. During the pause UM has freezed a couple of times. So much that I need to start the joblist and terminate UM with loss of all tracks in the waitlist and restart UM.
I use UM version 5.1.6, Tractor Audio 2 and windows 10, 64 bit.