Still working on that door. Had a friend try it with a bonus of almost 250, nothing but broken picks. It still even probed as a difficult task. I guess you really do need around 300.
Just thinking here, perhaps we are all skipping a basic part of the quest. Talking to some NPC somewhere else to get a key to the arms rooms. Also do both sides of every gatehouse have a rack then? I have been going to the West Gate as noted in this thread but there is a locked door on both the north and south side (and I would assume on all other Gates as well).
As I noticed while standing outside the doors, there will be an event every now and then of a soldier coming up and clearing the rack of items. Even though there is no rack in that room.
Yeah it took me ages to find a thief to pick the lock for me, but I got one eventually, the door stayed open for 30mins+. I had 10 daggers/knives on me which I put on straight away, then I went around and killed anything I could find that had weapons and brought it all back. The door was still open when I finished. Just make sure that when you get the door picked, "open doors" first, and then move through, otherwise you will open it, go through, and close it again, and it will automatically lock. I got stuck in there the first time
i'm pretty sure the rack from the quest is located in the djelian guard guild, the door wasn't closed at all when i got there, when you enter the guild 1 e then 2 n, the rack is here. I haven't started doing the quest myself but the rack was already full with various weapons so i assume it's the right one.