Here’s an annoying TMG gotcha: You update a web access / firewall rule to allow or deny access, but it doesn’t work.
The fix is to give it 10 minutes after clicking the apply changes button (Don’t forget that once you’ve changed the TMG rule, you have to click the additional Apply Changes button that pops up on the dashboard).
I experienced this whereby there was a rule denying access to a website, I changed the rule to allow access on the TMG server, but I still couldn’t access the website. I suspect this has something to do with caching, but after 5-10 minutes I was able to access the website. Yes, I did try deleting the IE cache with no luck. It’s definately a TMG feature.