TMG Error Code 502 Proxy Error. The directory name is invalid. (267)

Hi,

I am having problem with a website in specific.
Is showing the following error message.
Error Code: 502 Proxy Error. The Directory name is invalid. (267)
Server: TMG.personaldomain.local
Source: web filter

I have found no information in the logs to help identify the problem.
I suspect that may be temporarily unavailable from the website. I enter the site without using the TMG and the same is responding.
The problem is solved when I restart the service "Microsoft Forefront TMG Firewall"
I suspect that may be cached in TMG that the site remains unavailable.

Any idea?

Tks.

MCITP|Enterprise Administrator