Unfortunately, we're impacted by another Facebook bug for 30 minutes: https://developers.facebook.com/bugs/2456005991290482/
Because of this issue, you might face the 2 same problems regarding your Facebook private messages:
- Some private messages are not synchronized at all
- You're getting the message "Received Facebook error response of type OAuthException: (#1) An unknown error occurred (code 1, subcode null)" while you replied to your private messages.
We're more than sorry for this very annoying bug, beyond our control again :/
Be sure we keep you posted when we have more information.