I've used the same save file a few times and no matter where it happens if I hit level 20 on the assassin it will pop up with the screen to promote him but clicking on either new class will have it give the "You can't do that sound" and do nothing. If you're in the camp it will soft lock the game, in battle you could still at least restart and select something other than the Assassin to avoid being locked
Thanks for the report! I believe this should be fixed in the next patch, coming tomorrow. :)