Yo, I had this issue last week! The "facebook app session expired" thing is so annoying. What worked for me was logging out of ALL devices (even the ones I forgot about lol) and then logging back in.
Also, try updating your app if you haven’t already. Sometimes it’s just a buggy version. Hope that helps!
Also, try updating your app if you haven’t already. Sometimes it’s just a buggy version. Hope that helps!