We are very happy to let you know that Facebook confirmed that the incident has been resolved: Replying to Private Messages is now working.
==> https://developers.facebook.com/support/bugs/484143269217803/


Please note that replying to people that got in touch for the first time during the incident (between approx 10:00 AM and 11:00 PM, December, 2nd) is still not possible. Users would have to start a message with your pages again before you are able to send them messages.

Thank you for your patience while we waited for this to be resolved.

Issue seems to be partially resolved, we’re still monitoring the issue with Facebook, here: https://developers.facebook.com/support/bugs/484143269217803/

For now, you can :
- Reply to people who send you a new message
==> for new conversations (people who send a new message to your page for the first time since 11 PM, December 2nd)
==> for previous conversations: even if the previous messages have been impacted by the bug, it’s possible to answer them since 11 PM, December 2nd

But you still can’t :
- Reply to people that got in touch for the first time during the incident yesterday (so between approx 10:00 AM and 11:00 PM, December, 2nd)


We apologize for any inconvenience and will keep you updated on the status of this issue.
Thanks for your patience.

Facebook are currently experiencing an issues with replying to Private messages, showing this error (#551) This person isn't available right now

Facebook have a ticket on this issue here : https://developers.facebook.com/support/bugs/484143269217803/
We apologise for any inconvenience and will keep you updated on the status of this issue.