There can be multiple reasons for it , from my perspective following could be the one:
Since you did a database backup there might be some entity data validation which is failing .
For example Scenario :
You have a entity , Student created where there is attribute Roll no.
When you first created it there was no validation rule on that attribute and ran the application.
Then you added a validation rule on entity attribute that roll no must be unique.
Then you did a backup of data where student table got duplicate roll no inserted in the database .
Then when you try to run the application it fails as mendix performs a entity validation and see there should
not be duplicate roll no and since it does not meet the requirement you get M2EE error.
You can try to see the M2EE log there it might have shown where or on which validation rule it is failing.
Hope this helps!
Sunit Kumar Dubey