Sorry, guys!
But now, it seems to work!!! After I did the last post, I started my server and I recieved an update including one for zentyal-network to version 3.0.2. I don't know, if this was the solution, but now, when I tried to join the domain, at first, I got a totally different error message concerning a problem with my entered DNS servers. In the past, I not only entered Zentyals IP address as DNS server, but also that of my router. That might be wrong.
After I deleted the IP address of the router in the network configuration of my Windows box, I could join the domain as I expected.
I tested to leave the domain and joined again, no problem.
My latest test was to delete the last fixed entered DNS server in the network adapters configuration, too, to let windows get all network informations via DHCP and this works too!
Maybe, there is somebody out there and has the same expieriences?
Thanks for reply to all,
Ralf