From CONF-16131, which is caused by this problem.

RemovePagePermissionAction, for remove page permissions via one of the View Info screen's "open padlock" links, is not a SpaceAdministrative action. When the remove succeeds, the user is redirected to listpermissionpages -> ListPermissionPagesAction which is SpaceAdministrative.

Thus, if the user has permission to remove a permission on a page but not space admin permission, they will see the "Not Permitted" screen (e.g. for me, https://extranet.atlassian.com/pages/listpermissionpages.action?key=PM).

The remove success either needs to redirect to a screen the user has permission to view, or (preferably) use AJAX to perform the removal without leaving the View Info screen.