Seems the Trophy is not possible for Players with less Pick Lock Stats. In the whole Game allways be different Ways to open Quest relatet Chest by stolen Key and so on. But in this Quest all this should not work. There seems no way to get an Bag of Gold without these high Lockpik Stat. Im very angry about that after spending so many Hours in the Platin Trophy.
The chest is an easy one right? Try to get padfoot potion to boost your skill.
No it’s a hard chest. Just practice on very easy locks until level 3
Seems to be the way 
Thx Guys
Ok got it. Not a bug but time burning. You need to praktice Lookpicking for a while (takes me 2 Hours :() Best is a minimum Level of 6) you can also using a Potion makes you better in Lockpiking for a few Minutes.