Support has seen instances where users settings related to an App. can become corrupt. The quickest way to rule this out or get through the issue is to leave the exiting page intact so that other users continue to have it available. Create a new shared page with the same app and have the user(s) who are having problems with the original page, test the new page. If the app works on the new shared page then unshare the existing page and rename the new page to the same name as the old page so users see the same option. Engineering can investigate the issue, but it can be a lengthy process to determine the cause, a fix and delay a resolution. This is a fast track solution to get users up and running as soon as possible.