Thanks to Avid - I did it finally - I upgraded my Eleven Rack.
What I have done? Here I share my Knowledge - Maybe it is usefull for other users as well:
I got my old Windows 7 PC running again - real slow
Then I uninstalled Windows Explorer like asked by the AVID support.
"You have mentioned having version 11, but you might be also having Internet Explorer 9 there, so please check and uninstall that as well using the same instructions here:
http://avid.force.com/pkb/articles/troubleshooting/en394771"