Getting error "The XML element 'EnableTheming' from namespace...." When running Web service

Hi everybody i am getting an error

The XML element 'EnableTheming' from namespace 'http://tempuri.org/' is already present in the current scope. Use XML attributes to specify another XML name or namespace for the element.

when i am running my webservice There is no error at build time its build up successfully.

I an getting this error when i return an asp.net pannel from my web method.. but when i am returning simple string its work fine..

Thnks in advance,

Raj