When trying to view a webpage, you TMG gives error 502:
Error Code: 502 Proxy Error. Forefront TMG denied the specified Uniform Resource Locator (URL). (12202)
Explanation: There is a problem with the page you are trying to reach and it cannot be displayed.
This error code is a little confusing because it says that TMG denied the URL, when actually the friendly explaination is actually correct – There is a problem with the page you are trying to reach. TMG gives this 502 Proxy Error when the website you are trying to reach is down!
You can confirm this by using another network connection (such as your mobile phone) to try to reach the website.