I get your exact error as well.
I can also get this error.
I've managed to get inside the western boss room 1 time, and kill 3/3 of the bosses.
And, I've was able to get inside the northern boss room 1 time, but, crashed to desktop when I engaged the 3rd boss; martel I think.
I've tried to enter the 3 boss rooms probably over 30 times over the course of the month.
As for legion.
A few nights ago, a group of us, 4-5 players, entered legion's room and we all crashed... when mobs started spawning. We also were all slaughtered... meaning our chars were still in the game, but, our clients crashed.