Hey, I had the same problem, and it was because of my antivirus software. It was blocking Facebook’s cookies for some reason. I added Facebook to the exceptions list, and the "why is Facebook saying session expired" error stopped.
If you’re using antivirus or a firewall, maybe check the settings and see if Facebook is being blocked.
If you’re using antivirus or a firewall, maybe check the settings and see if Facebook is being blocked.