Completed the Theatre Troupe event (yes, I had the quest and received the chest)… it did not contain a coffer key. I had one coffer key before doing the event, and I still only have one.
Then I did the radiant shard quest for the Radiant Cache in Hallowfall. The game message said I received a radiant cache, but there isn’t one in my inventory. I’ve logged out and back in, and it’s still not there.
And NOW the “Spreading the Light” quest (which is the quest referenced above which supposedly rewards the Radiant Cache) seems bugged, because I activated four lesser keyflames, and then the count reset. I tried activating another one, and the counter is now stuck at 0/4 activated.