The quest is Bound by Pain, the one where you have to free a possessed dwarf. I've sung the hymn at the Blazing Sculpture, entered the vault, and used Spirit Vision to speak with the Brother.
I have a Silver Lever Shaft forged at the Ancient Forge. When I try to combine the Silver Lever Shaft with the Broken Shaft it does not combine, a question mark pops up instead saying unknown item.
I've done the exact same thing for the other vault with the possessed lizard and it worked fine.
Seriously, between this and Tarquin no where to be found I have to wonder whether all the reviewers giving DOS2 10/10 scores played beyond the Chapter 1 to see all these quest breaking bugs.