Had this happen to us last time, as you can see, the chest is completely encased, when it was closed it couldn't even be seen at all.
The abbot himself was building that tower around him and at some point couldn't hit him, or target with spells, just AOE.
I know the asteroids from the puzzle are supposed to be used for these, but unfortunately the ones who completed the asteroid puzzle died during the fight and lost them.
Here comes my suggestion to change them from destroyed on death, to drops on death please pretty please.
We did manage to loot with targeting backspace, and using U to open but was pretty frustating and had to trial and error till we found just the 1 inch spot we could use it on. Changing the quest items like that would save a lot of trouble, not just this one specific but other issues and possibly make the raid just a bit easier maybe.
Please Turbine look into this one, if it is an easy change.
Thank you for reading, regards: Vivanto