So until now there is no solution. Steam support is not able to help. They had some suggestions i already had done before.
Now they directed me back to this forum.
In addition i deinstalled and reinstalled Tower2011:SE again and also have checked my Steam client and game updates.
But no online connection, no game play. I have to mention that all other Steam games i purchased (Tower!3dPro for example) are nicely running offline as well.
I am little bit disappointed that i have to deal with two partners (Steam and Feelthere) to get my issue solved, but no success.
Please dont get me wrong, if a game has to have an online connection and this is stated before, thats o.k.
But in this case i would not buy it.