We were getting the below error
<description>An unexpected error occurred.</description>
on our custom workflow activity for the following requests
This is how we were creating the CrmService instance
however later we changed it to
and the error got resolved.
Both of these methods return an ICrmService interface. This is optimal way to interact with CRM to execute the various requests in the CRM web service. The overload allows you to execute with more privileges if needed; just in case the current user doesn’t have the required privileges to perform some action in your custom business logic.
More on IWorkflowContext interface