Registered to share my experience with these chests. I've been hunting them on 6 chars.
1st char: Looted 1st chest and found a fortuitous coffer, but no key or ritual stone. Looted 2nd chest a few minutes later and found a key. Seen no chests since then. (Wednesday)
2nd, 3rd, 4th and 5th char: Got key from 1st chest. Seen none since.
6th char: Same as 1st only this was today.