The "Session Expired" error usually happens when the app's cache doesn't match Facebook's server data. It's a security feature that sometimes gets stuck in a loop.
Clear App Cache & Data
Corrupted temporary files are the #1 cause of this loop. Clearing the cache forces the app to request a fresh session token.
Remove Account from Device
Go to your phone's system settings and remove the Facebook account entirely, then log back in through the app.
Check Date & Time Settings
If your phone's time is off by just a few seconds, Facebook's security protocols will expire your session immediately for safety.
Update via Play Store / App Store
Older versions of the app often have bugs with new security certificates. Ensure you are running the latest version.
Change Your Password
Changing your password from a web browser will force-close all active sessions globally, which usually resets the loop on your phone.