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!
For anyone still dealing with the minecraft blocks for T1 Walls. I've created a mod called 'T1 Walls (Vanilla)' and released it on the FAF client.
This mod will restore the original T1 Wall sections into your game for immersion purposes.
This is a work around for now, I'm sure the April fools blocks will be removed at some point.