Unable to connect to graphic server

I keep getting this error message when trying to connect to a VDI machine (see attached screen shot):

"Unable to connect to the graphic server"

RHEVM environment is RHEV 4.0; RHEL 7 workstation virtual machines; the virt-viewer client is downloaded from the RHEV manager

This is a new environment. Initial testing was done on a network local to the RHEV manager; everything appears to be working correctly in that environment.

The issues are encountered when users on an external network try to access the VDI machines. The attached screen shot shows what they are seeing - this error does not occur on the network local to the RHEVM.

Our firewall guy doesn't see any traffic being blocked/dropped by the firewall.

Not sure where to go from here.

Paul