Cannot connect to TFS from VS 2015 Enterprise

Hello,

we run a TFS server. It works fine. I use a Laptop (Win 7) connected to our enterprise NW using VPN. In the past I could easily connect to our TFS server over the VPN connection from inside VS. Now I installed VS 2015 and now I cannot connect to the TFS server from within VS. All other connections to server over the VPN work fine. If I open the TFS URL (https:/<TFSServer:Port>/tfs in the browser it works well. I tried the FQN and the IP address in VS - does not connect.

The connecting indicator keeps spinning without an error message forever.

I have another machine also with VS 2015 directly connected to the enterprise NW where it works fine (that is a Win 8.1 machine)

Any idea?

Thanks for your support in advance

Hansjörg