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
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java.util.concurrent.CompletionException: java.util.concurrent.ExecutionException: org.springframework.web.client.ResourceAccessException: I/O error on GET request for "https://api.faforever.com/featuredMods/6/files/3719": Connection reset;