Is it possible for the user to see the Case in a different way, such as in a listview, a report, or by searching for the Case Number in global search? Normally, instead of a blank page, you'd get a significant error.
Perhaps the sharing rules need to be recalculated? If they get stale, go to Setup -> Sharing options -> Case and click the button. Is your user record id ("005...") or "FirstName LastName" referenced in your sharing rule?
Check what's in the [Sharing] button if you switch the UI to Classic and go to the case record as an admin (it might not be present, in which case add it to page layout). You should be able to see high-level information, as well as the opportunity to expand the view and see who can see/edit this record (at the highest level of access) and why. Use this to play about with the sharing rule and see the results as an administrator without having to log in and out.