"Users were messaging via the web frontend, so we fixed the glitch."
They just simply decided one day, oh hey let's redirect the messages page on the Facebook app to a static page saying "download messenger" and not give any access to the messages, including view-access. Even when you get a notification on your phone about a new message.
FWIW, I discovered this because Messenger would not show me a group chat that I was added to by co-workers (who I'm not friends with on FB).
Edit: just realized you were referring to the app, not the site. My mistake.