I have had the exact same problem after migrating from 2003 to 2008 R2 and found the solution. Change:
Set objhttp = Server.CreateObject ("MSXML2.ServerXMLHTTP.6.0")
to:
Set objhttp = Server.CreateObject ("MSXML2.XMLHTTP.6.0")
and your problem will go away.
I tried to find the pros and cons about both objects, but haven’t yet found a reason to not use XMLHTTP.