Tonight I uploaded a mission to the custom arenas. Whenever I tried to assign pilots using the Populate Mission screen, I would get a Host Connection Lost and CTD. Upon re-entering AH a couple seconds after, my custom arena was already deleted. I'd say it took 15-20 seconds from CTD to being back in and seeing the arena no longer listed. Mission runs fine offline. Tried it 4 times with same results every time.
