One issue off the top of my head would be the Tasks.
If you have ITSM configured to not allow an Incident to be Resolved with open tasks then that will be a problem.
If you do not have this option configured and you close all your incidents you will have open tasks.
Check your configuration and I would setup an escalation to take care of any tasks before I created an escalation for the incidents.
The other possible option is if you changed any groups / assignees, i.e. disabled them or deleted them the system will give an error if you try and resolve or close an incident as it 'checks' these items when the incident is being updated.
Hope this helps.