User is unable to edit the settings for apps on a shared page even though they are a Project Administrator.
Shared pages can only be edited by Project Administrators with admin permissions to every project the page is shared with. If it is shared with a project the project admin doesn't have access to then they will no longer be able to edit the page even if they were able to edit it in the past.
You could make the project admin an admin for every project that the page is shared with or change which projects the page is shared with.
The other option would be for the admin to make a copy of the shared page for their own personal use. You can make a copy of a shared page by clicking the gear option in the upper right of Rally.