Why was Martin suspected?

Before Martin and Jens begin their competition with "Who dares..", Jens tells Martin that he have *beep* a hooker named Joyce. But the important thing is that Jens told Joyce that his name is Martin. Therefore, naturally, Joyce mixes the friends around.

So when Joyce tells Martin's girlfriend, Kalinka, that her boyfriend is a sick bastard she really means Jens. Thats why Kalinka gets so upset because she also doesn't know about the game Jens and Martin is running.

When Inspector Wörmer is trying to set up Martin in Joyce's apartment he writes "Martin" on the pillow with her blood so that the police thinks he is the serial killer. But again, Inspector Wörmer isn't aware of the game with the names. So when the police talks with Jens he tells them about how the names was missunderstood.