The most common cause for this error while accessing all HTTPS web sites is because the client workstation doesn’t trust the certificate that TMG is using. The CA certificate (e.g. self signed certificate) used by TMG must be deployed on the client, otherwise the client won’t trust the certificate issued by TMG on behalf of the web server. Read Deploying the HTTPS inspection trusted root CA certificate to client computers from TMG Documentation on TechNet for more information on how to deploy the CA certificate to the clients.
Microsoft® Forefront Threat Management Gateway (TMG) 2010 Service Pack 2 (SP2) introduces new functionality to Forefront TMG 2010 Standard and Enterprise Editions.
The service pack includes the following new functionality and feature improvements:
• The new Site Activity report displays a report showing the data transfer between users and specific websites for any user.
• A new look and feel has been created for error pages.
• Error pages can be more easily customized and can include embedded objects.
• You can now use Kerberos authentication when you deploy an array using network load balancing (NLB).