I have given a user group 'Edit' and 'Edit State' permissions on a page's permission settings.
Calculated Setting for this user group in now 'Allowed'.
Then I assigned a user to this user group to enable editing of this single page, but not any other pages.
But when the user logs on the site in the front-end and tries to open the editor, this error happens:
403
You don't have permission to edit this page.
Only when the user is assigned to the 'Editor' group is the editing allowed. But then, the user can also edit all other pages, which is not acceptable.