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!
Error lua script function play
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warning: Error running OnRelease script in <deleted object>: ...mdata\faforever\gamedata\lua.nx2\lua\maui\button.lua(101): Game object has been destroyed stack traceback: [C]: in function `Play' ...mdata\faforever\gamedata\lua.nx2\lua\maui\button.lua(101): in function <...mdata\faforever\gamedata\lua.nx2\lua\maui\button.lua:94>