I could reproduce it 3 times out of 3 times, as a leader in a group of two and going into the 2 player pve battleground and trying to change the difficulty from 1 to a higher one. It crashed after that with the same assertion failed. I was the group leader at that time.
Edit: after testing with kubik i found out: If the other player is on another map (like forge) it will crash, however if both went to battlegrounds, it wouldn't crash anyone.