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!
I am getting this error as well. No joy with the above fix @magge
I am on windows 10, have norton but disabled everything to attempt it. Have deleted game.pref, all my mods, the game itself, the launcher, restarted comp, moved it to different drives, and even did one final try with admin