Facebook calendar URL getting error about 30% of the time

[dribnus]

There are similar discussions but the don't see to be exactly like this issue. I have my Facebook event URL in GAS-ICS-Sync. It works about 70% of the time. When it doesn't work and gets an error, all events are deleted until the next run. It says "Incorrect", but it still works most of the time. I'm not understanding.
[ERROR] Incorrect ics/ical URL: https://www.facebook.com/events/ical/upcoming/?uid=1391956594&key=AIadY6th0BPNF%%%
last 3 characters edited for privacy.