Error while adding custom workflow activity to the process

Hi, I am getting the below error while adding the custom activity step to the process.

Registered the assembly and plugin successfully

Removed all my from the code, kept only the initialization of service stuff, still getting the error. I think it is related to versioning issue.

I am using VS 2012, CRM 2011 on premise. Below is the error log. Any help is appreciated.

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Unsupported type: System.GuidDetail:
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147220715</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>Unsupported type: System.Guid</Message>
  <Timestamp>2013-05-01T14:38:02.7613115Z</Timestamp>
  <InnerFault>
    <ErrorCode>-2147220970</ErrorCode>
    <ErrorDetails xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
    <Message>System.NotSupportedException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #5546E420</Message>
    <Timestamp>2013-05-01T14:38:02.7613115Z</Timestamp>
    <InnerFault i:nil="true" />
    <TraceText i:nil="true" />
  </InnerFault>
  <TraceText i:nil="true" />
</OrganizationServiceFault>