I assume you are able to open the scenario in the editor.
What scripts have been edited?
What changes have been made to country data?
Are you able to Save As the scenario to a new scenario and edit that?
I'm stabbing around in the dark here but there are a lot of things to troubleshoot.
With script editing, you can get some idea of an issue when you compile the scripts before saving the campaign.
Go to Campaign\Edit Event Scripts
Choose the script and click the 'Update' button
If there is a problem, it will give a description at the bottom of the dialog box and a line number.
If you use something like Notepad++ or any text editor with line numbers, you can go to the script and troubleshoot it until it compiles properly.