Actually we had a completely different run.
When the Archdiviner went into the first stun phase, no mobs spawned which caused us to die and reset it.
When we went into the fight a second time, he did not go into a stun phase, and when we got caged, the cages were not targetable, and we ended up dying.
When we went into the fight again, it seemed normal and we managed to attack the seal down to 0 HP. However, the mobs still kept spawning, and the event did not end.
This event seems to be royally bugged.