Spent quite some time trying to figure out this error message
This happens when i tried to send http POST to a remote computer, work fine on my local machine, from here it says.
The .NET Framework 1.1 defines a new protocol that is named HttpPostLocalhost. By default, this new protocol is enabled. This protocol permits invoking Web services that use HTTP POST requests from applications on the same computer
the solution is to add the following in the web.config