Today i was tasked with debugging and fixing a custom web service that interacts with sharepoint. seeing that the web service only actually required a few strings as parameters i decided that the best way to test the service was to pass the parameters to the web service method via the URL.
My first attempt product the above error.
This issue was solved by enabling the following in the applications web.config
<configuration>
<system.web>
<webServices>
<protocols>
<add name="HttpGet"/>
<add name="HttpPost"/>
</protocols>
</webServices>
</system.web>
</configuration>
This is because as default, the HTTP Get and HTTP Post protocols are disabled (therefore disallowing consuming the web service through the URL)