Ok I changed the diffusor with three more basstraps on stands (there's a window so they cannot be attached to the wall). They are 10cm thick and 30 cm away from the wall.
I calculated the RT60 time and the following graph is the result.
It looks ok to me although I really hate rooms that sound too dull. The High frequencies are suffering quite a bit under all the basstrapping. Am I just taking this graph too serious or should I change some of the basstraps in the back to paneltraps or helmholtz resonators to improve the sound?
Any suggestions are more then welcome.