(Changed Character to be able to post)
I think I have the solution even though I havent been able to try it yet.
Reading on the US forums about the subject, one person suggested that if you are in the cave when the event starts you dont get thepublic quest. What you have to do then is to fly out and then fly in again to get it. Without the quest you cant loot the chest in the end.
This has actually happened to me before ina similar event. When I parked a couple of characters in the Iskara village for the event and I parked them near the couldron. Logging in, even if I sat near the cauldron, I didnt get the event quest so I had to fly away and then back to get it.
Perhaps there is a similar bug working with this event…
I have now Tried it and it worked.
When the event started and I didnt get the event quest/counter, I flew away a bit and then back and the quest started. Wen the event was done the chest was lootable.