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!
@Sir-Prize Yes, I was looking for this error and could not find a clear answer to this question. It seemed to me that I began to meet this error more often when I slightly overclocked the processor. Perhaps this also leaves its mark. I will try to follow your advice, I hope it will help. Thanks for the tip!