When you create a new page on Pages and then change its path to something different, the routeId doesn't change. So when you create another page with the same path as the first one the routeId will remain the same. And then the site editor will have problems saving the content of these two pages due to conflict on their routes.

1 - Create a new page 2 - Change the path of this page 3 - Create another page with the same path as the first one (the original path, before you change it) 4 - Try to make changes on those pages in the site editor

Delete the older path and try not to change it again. If you need to change it, it is best to delete it and create a new one.