I edited my question from yesterday: django authenticate() allways returns None, but I think noone will will pay attention because I already marked the question as answered. - a
Now authenticate() works. It was a version-mismatch. Now I remember that I created the initial project at home and on my gentoo-machine there still is the version 1.7.7 installed and on the windows-machine at work 1.8.4., I think it is the different settings-file, stupid mistake from me.
Sorry for the inconvenience