Possibly this page is still used in another navigation profile. If you go to Navigation, below the button ‘Add navigation profile’ you'll see two tabs by default. Make sure this page is not used in any one of these tabs. It can be set as default homepage, but also as Role-based homepage, make sure to check that.
Hope this helps!
I only know of one situation:
- Create PAGE1
- Enable Anonymous users
- Set login page to PAGE1 in navigation
- Disable anonymous users
- Remove PAGE1
Page1 is still used on the hidden setting for anonymous users, so Mendix complains about it not existing anymore…
I guess there are more ‘hidden’ settings that behave this way.