When you first user admin pages creator in uipress, occasionally when trying to view the pages you can get a 404 error. This is because admin pages use a custom permalink structure.
In order to fix this issue all you need to do is re-save your permalink structure in the WordPress settings.
From the main menu go to settings > permalinks. Once there you don't need to make any changes, you can just press the save button towards the bottom of the page. Once saved your admin pages should be working correctly.