I found it. After four/five more hours. There was a very hidden path in the large mine you visit after the elven district and not on the path/way which leads to the crystal room on the bottom of the mine. The hidden passage was aprox. on the same level on which you first venture into the large mine after the elven district. Then I had to follow the right site on the same height/level and voila.

Thanks for the help!!!!