The issue we faced with PM's has now been resolved. You can now take up to 7 days to reply to PM, in accordance with Facebook Policies https://developers.facebook.com/docs/messenger-platform/policy/policy-overview
Thank you for your patience while we resolved this situation.
We are experiencing a technical issue with Facebook PMs. You may see an error when attempting to reply to a Facebook Private Messages older than 24 hours. We are currently looking into the issue and appreciate your patience.