There seems to be a built in limit for non-rogues in NWN. We cannot pick any lock or remove trap at a dc higher than 35. Its hardcoded as far as I know.
I am not sure for bards or the various prestige classes, but otherwise you need at least one rogue level.
Playing a cleric with divine trickery and points spend in rogue abilities this came as a rather unpleasent surprise.
The same problem mentioned with the chest also goes for the door leading into the "throne room" with the leader. In this case its open lock thats the issue though.
Edit: not sure about rangers and remove traps.