sites were unaccessable and displaying Error Code: 502 Proxy Error. The data is
invalid. (13) Source: web filter
destinations web server is sending responses back in chunks which is a common
thing and if configured correctly does not cause any issues. However TMG's HTTP
compression is seeing these chunks as seperate requests and cannot piece them
together (search Chunked Transfer Encoding for more information).
TMG is functioning correctly! The issue is most likley that the desitnation web
servers HTTP headers are incorrectly configured. I would attempt to contact
the sites web administrator and point them in the direction of checking their sites
Transfer-Encoding header is 'chunked'.
If you need to get round this error, you can either bypass TMG or disable
HTTP compression on TMG.
To do this, open TMG > Web Access Policy. Under Web Access Settings, click 'HTTP Compression' and deselect 'Enable HTTP compression'.
Though not ideal, you will now be able to access the site(s).