Solved! Go to Solution.
If you are using 10.0 or 10.1:
http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#//00460000002w000000
If your web service is x64 then you have to bind and license to ArcGIS Server. Desktop is strictly 32 bit, i.e. x86.
I suspect that you have not configured your web service to run as x86, i.e. 32 bits only.