Hi there
Have you tried another form of viewer, for example try:
www.realvnc.com and connect to the workstations with that viewer instead. There is a free version that should be fine for testing here.
Also, I know RealVNC has port 5900 for TCP and 5800 for a web based viewer, is it worth trying that too, to see if the page still loads black?
http://ipaddressofworkstation:5800