Password protected posts not found

Hi! I’m having trouble suddenly with password-protected pages on my site. The linked page is an example of a page that should be password-protected, ie. anyone visiting it should be prompted for the password. Instead, it just isn’t found at all, with no opportunity to enter a password. Can anyone advise what might have gone wrong or how it can be fixed? Thanks!

The page I need help with: [log in to see the link]