Hi,
I think there is a serialization problem you can find the exact error. Just add the below code in service config in section.
After adding the below code, a"App_tracelog.svclog" file will be created where your service exists. Open it and find the red colored line which is the error and see its description.
<system.diagnostics>
<sources>
<source name="System.ServiceModel.MessageLogging" switchvalue="Warning, ActivityTracing">
<listeners>
<add name="ServiceModelTraceListener" />
</listeners>
</source>
<source name="System.ServiceModel" switchvalue="Verbose,ActivityTracing">
<listeners>
<add name="ServiceModelTraceListener" />
</listeners>
</source>
<source name="System.Runtime.Serialization" switchvalue="Verbose,ActivityTracing">
<listeners>
<add name="ServiceModelTraceListener" />
</listeners>
</source>
</sources>
<sharedlisteners>
<add initializedata="App_tracelog.svclog" type="System.Diagnostics.XmlWriterTraceListener, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" name="ServiceModelTraceListener" traceoutputoptions="Timestamp" />
</sharedlisteners>
</system.diagnostics>
Hope this helps you in finding the exact error :)
Regards,
Praneet