This was a funky bug I encountered today, I was playing as a Meteor Goliath with my friends and my game crashed to desktop with no error. I fired the game back up and rejoined them as fast as possible (possibly what caused the bug to occur perhaps?) but when I got back into the game my ui and all monster FX was gone.
No mini map, no blue fire on the goliath, invisible flame breath, invisible rock throws and the worse of it all? There was no evolve menu to select my skills so I had to randomly swing my mouse around randomly clicking to try to evolve.
Oh and also I was unable to evolve to stage 3 for some reason, only the evolve to stage 2 worked. The press v to evolve still worked and my energy bar was filled up to full, the only UI that was still functioning.