Thanks for the reply Tanner.
I have taken a closer look to the resistance cable tension. I had the feeling that something was blocked internally. So i have opened up my tacx, removed the flywheel and noticed that the magnet (which position is adjusted by the resistance lever) was no longer sliding up when in postion 1. Meaning postion 1 was actually position 6-7 or so… I greased it up to make it work again.
I just did my first ride. It works like a charm and i do no longer have to give a max effort to reach 25km/h 
Thanks for your help!