SharePoint 2010 - An exception occurred when trying to issue security token.

Make sure that "Claims to Windows Token Service" is starte if you see the following error in the event log with SharePoint 2010:
Task Category: Claims Authentication
Source: SharePoint Foundation
Level: Error
EventData: The server was unable to process the request due to an internal error. For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework 3.0 SDK documentation and inspect the server trace logs.