Thanks Ryan, after changing the session name I can login. If I change it back, I cannot. I assume that database sessions will solve this problem, but to install that module I needed to login first .
Ok, a final comment now I have found the real issue. Using db sessions makes no difference, using 'wire' instead of session_name() still triggers the problem. After reading a comment on php.net about session_name() I checked my php.ini and session.auto_start was true. I don't know if I did that myself or it comes with a php update on debian, but setting it to false and restarting apache solved the problem of this topic.