One thing to look at when you are unable to sign in from a desktop product is that your system date, time and country region are set to where you are located.
Also be sure you are running the latest service pack for Revit as that did resolve some Render as a Service issues.
If you have a proxy server and firewall that could also be part of the issue. You would want to make sure not only the ports are open but that you have access to the sites listed below.
*.akamaiedge.net
*.amazonaws.com
*.autodesk.com
*.google-analytics.com
*.cloudfront.net
*.getsatisfaction.com
Firewalls should allow ports 80 and 443 to everything if possible.