Under the Duchal Palace is some kind of treasury. There are several guards and a big treasure chest. I have tried to open this chest many times but it didn't worked. Even with 160 lockpickin'I couldn't get the damn thing open. Does anyone have a clue?

A lot of thanx, Philippus

Where exactly is this treasure chest of yours?
But more to the point, whenever I'm faced with an 'unpickable' chest, I use the potion of Strength (25) obtained from one of the carnival tents in Nashkel (other Strength potions should also work) before forcing the lock open (click on the 'Attack' icon then click on the chest). Theoretically, you can also try the 'Knock' spell, but I couldn't get it to work in practice - anybody has any ideas?
Hope this helps...

I've opened it once, but there isn't much of interest in it, just some gold, jewels and some armor (nothing special/unique). I think that without TotSC the lock requires a special way of opening it, but with TotSC you can just open it using Pick locks.

BTW, you can enter this room by using the stairs behind Sarevok in the throne room of Duchal Palace. Just go back there after the ceremony (the invitation still works). The upper levels offer some treasure too...

Petrarcus ap Bhaal.