I saw this error some time ago while working on an eCommerce solution. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server. Redirect/move happens like it should, but after approximatively 2 minutes after posting an order the user receives an http error 502.
The order is correctly processed by SAP but the web applications is built in a way that the user is getting a confirmation page when the order is processed: this page is never presented to the user due to the 502 error.
FREB gave us further details about the error: 502.3, 2147954417 which translates into ERROR_INTERNET_OPERATION_CANCELLED.
This lead me to this post which describes a different error but based on the same principle and the solution still applies to our scenario: we increased the proxy timeout to 5 minutes to accommodate all reasonably long running requests and the problem was solved.
appcmd.exe set config -section:system.webServer/proxy /timeout:"00:05:00" /commit:apphost
It's not the size of the dog in the fight, it's the size of the fight in the dog.