Getting http response exactly after 3.8 minutes with status code 500 OK

I'm sending a http request to a web app which is hosted on Azure, the request needs more than 1 hour to complete. After I send the request in Chrome, I use F12 to check the Network status. The request returned with status code 500 OK every time, and the time used is exactly 3.8 minutes.
I have remote debugged this web app and find out the code is running perfectly fine, and there is not a single exception indicating the 500 OK error, but the browser just returned 500 OK.
I have also googled a lot. But due to the strange status code "500 OK", I couldn't find anything useful. 3.8 minutes appears like some time-relevant settings, so I tried setting executionTimeout in httpRuntime tag in Web.config, and also tried ServicePointManager.SetTcpKeepAlive, neither of these work. Has anyone come across situation like this?