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!
@GANY
just in case you need to post a log in the future:
Go to the webpage www.pastebin.com and post your log there.
After clicking on "create a paste" copy the new pastebin web URL.
Then you only need to post the URL here.
example URL:
https://pastebin.com/VnpaPec9
"VnpaPec9" is the ID from my last test paste.
You will get another ID when you paste something
Thanks a lot guys!
Should have looked here before closer, but I am just not enough into coding or have not read it through well enough, so that I got it where the mistake was coming from...