Hm I don' know why but it's working now?!
We just bought 2 new T520's for another customer and I wanted to test it again. And now it's working fine, I deployed the basic ThinPro 6.1, made my configuration, captured that image and deployed it to another T520.
Thank you, and sorry for the trouble. I didn't change anything and I don't understand why it's working now.
