So being SAM61 with Haubergeon, Chivalrous and so on, I decided to open this coffer just for the looks of the piece. I already had a coffer key that dropped on ZM4, so I went there as PLD75 to check coffer spots. First, the Guardian killed me with Meltdown, and I was able to confirm that the coffer was behind of the Uggalepih door (you can notice it by pressing tab key). A ls fellow needed help with G1, so I decided to try it again the next day. I went to the Temple again like 10 hours later, and the coffer was still there. I started to farm for Uggalepih keys, but not a single drop after 2h+ of farming. Then, suddenly, the coffer spawned in front of me, in the room with the doll and 3 tonberrys. I warped back to Aht Urghan, changed jobs, bought some prisms and took outpost warp to Yhoator Jungle and bingo.
Just take care with the 3 tonberrys if the coffer pops in that room.
So this coffer is a pain in the ***, one of the few that is not soloable if you have bad luck and spawns behind Guardian, unless you go there at lvl 75.