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!
@sgtray I was running into a crashing bug, just after the last update. People suggested trying to watch an older replay, guess the game will get updated to the older version/patch. Seemed to work but could only watch replays. The it turns out that patch was reverted and rolled back. After that i could play again...