Why does net.socket.connect not work under Win10 (working under win7)?

Question

using this code

IPEndPoint remoteEp = new IPEndPoint(ipAddress, port);
this._sender = new Socket(ipAddress.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
this._sender.Connect(remoteEp);

to connect to an other computer works fine under win7.

With Win10 it throws an error:

Translation: No connection could be established because the target computer refused the connection.

Any idea how to solve the problem?

Edit: The firewall is off, ping is working, I use a distinct IP4-address, server ist running (and answers to a win7-machine properly).

With thanks in advance Peter