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!
Thanks for the directions. With the download I was able to identify the issue: there were still some files from a very old version of FAF still around, including an older version of the executable of the game, and the client seems to have been trying to start that one. Fully deleting the older files has resolved the issue.