The latest error codes that can be returned are available here: https://[Operational or Test Environment URL]/resources/doc/validation_en_US.properties  The following document will walk through issues that people have had in the past and what the fix was.

Webservice.CannotCancel/Webservice.CannotClose

It is important to remember that there is a distinction between cancelling a flight plan and closing a flight plan.  If you have activated a flight plan, you can no longer cancel it, it must be closed.  Likewise, you cannot close a flight plan that has not been activated, it must be cancelled.