Zalmarath became (temporarily) untargetable/invincible after recovering from a crash.

I was fighting Zalmarath, the Colossus, when my game crashed shortly after starting. After rebooting and resuming the fight, he did his side-stepping-fireball-spam attack, and became permanently* untargetable/intangible/invincible. None of my projectiles land, my curse AoE does not affect him, and my SRS minions are not going after him.

*Update: After doing that same attack again (the side-shuffle while spamming fireballs), he became targetable again.

This is happening to my friend and I in hardcore and we are basically softlocked on this boss out of fear.

EDIT: My friend reports removing Skeletal Reavers and Shock Mage and he no longer went invincible.