This happens (but might not be limited to) when there are two users who both have the 'Confluence Administrator and 'System Administrator' global permissions. These two global permissions are given by default to all users in the confluence-administrators group.

Steps to reproduce:

1. As a user in confluence-administrators (say userOne), create a new page
2. Set the page restrictions so viewing is restricted to userOne only
3. Log in as another use in confluence-administrators (say userTwo) and view the page you just created (you should have permissions to do this since you're sys admin)
4. Click the 'Edit' button
5. Edit and save the page - you will be taken to the 'Not Permitted' page
6. Go back to the page and remove the page permissions
7. Edit and save the page - your changes will be saved successfully

This behaviour is strange. Either, as a System administrator, you are allowed full permission to edit any pages OR if you can't edit a page, the 'Edit' button should not be displayed and you shouldn't be allowed to load the editor.

The behaviour remains the same for child pages as well.