Bug - Defeat Rathbreaker, can't respawn at checkpoint or through the main menu

Died to Rathbreaker

I tried using resurrect at a checkpoint, and the message received in the bottom left,

You do not have the required quest state.

Separate issue:
At the Rusted King boss (Act 1), my Sorc died. But before pressing respawn, the Rusted King died after a delay. It did not progress the quest to completion, and the boss remained unsummonable permanently.