I had been facing this problem when I tried to return a DataTable. This error 
was keeping on occuring what ever.

Here's the solution. 

Uncomment these lines in web.config.


      <jsonSerialization maxJsonLength="500">

        <converters>

            <add name="DataSetConverter" type=&
quot;Microsoft.Web.Preview.Script.Serialization.Converters.DataSetConverter, 
Microsoft.Web.Preview, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35&
quot;/>

            <add name="DataRowConverter" type=&
quot;Microsoft.Web.Preview.Script.Serialization.Converters.DataRowConverter, 
Microsoft.Web.Preview, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35&
quot;/>


            <add name="DataTableConverter" type=&
quot;Microsoft.Web.Preview.Script.Serialization.Converters.DataTableConverter, 
Microsoft.Web.Preview, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35&
quot;/>


        </converters>

      </jsonSerialization>