Same problem here.
We arrive at the Flooded Temple, but no cutscene, no dialogue, no mobs spawn.
Nothing.
We try to talk with Detha, /kneel at temple, teleport at the waypoint, kill some mob…
Nothing work.
Only change the description on the quest-log. Appears “Head at the flooded temple” just under “Go with Detha at the flooded temple”.
So sad: we waste 2 hour for it and we haven’t enough time to restart the dungeon again.