Hi Pi-L
looks like it is a Problem of the installed Xen.
I used another XENSERVER, which is just for test and lab pourpose, but basicaly not in the same IP segmnet. Putting it into an IP range which could be reached via WAN, Icould plug the open-e dss6-lite.

Now I need to find out whats wrong with this XEN Server, since this is the same installation build as the one which could do the plugging
Thanks for your help.