Can not connect to remote desktop from WAN

We have a test enviroment with 1 vWorkspace 7.5 broker and 1 RDS server, both 2008R2.

From a LAN based workstation I can connect to the RDS server through the vWorkspace gateway.

When connection from a workstation through the internet, i cannot start the remote desktop.

The error message is: this computer can't connect to the remote computer

Logon and downloading/installing the client is possible.

Firewall ports 80 and 443 are open.

Any ideas?