SSL VPN Portal and Website

We are using the VPN SSL Portal to Connect to Server and Website.

But we have some problem with intern Website, which need authenfication from AD.

We got allways the Message

"Secure Connection Failed An error occurred during a connection to rds.die-schwenninger.de.

The page you are trying to view cannot be shown because the authenticity of the received data could not be verified. Please contact the website owners to inform them of this problem."

We have actived Single Sign On in the Portal, Website which working without AD activation reachable and outgoing Website working, we are using Proxyserver.