The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Yep, I just did it and it worked! There was a C:\Program Files\Downlord's FAF Client\lib\downlords-faf-client-1.4.3.jar remaining from the uninstall of the previous version, I think it might have been responsible from the problem (?).
Thanks Sheikak, Rowey and Aska!