I ran into this AGAIN today. Of course it's been so long since the last time that I had forgotten the fix.
If you try and host Web Services on your Sharepoint server, it will fail horribly. And the only error you get is the very informational HTTP 500 can not load page. Nice. So I looked at a web.config for one of my older web services and found the missing piece! (Will need to remove extra spaces and the "!" add to allow post to show)
< !webservices >
< !protocols >
< name="HttpPost">
< /protocols >
< /webServices >
< !-- Set the trust back to Full. WSS configures a very restrictive trust policy that does not allow most applications to run correctly. -- >
< !trust level="Full" originurl="">
Addinig those lines of code should have you back up and running!
