We are currently experiencing an error when our customers try to respond to Facebook private messages.
The error code looks like:
Facebook error: (#33) This object does not exist or does not support this action (code 33, subcode null)
Replying to comments and publishing does not seem to be affected at this time, only Facebook PM's.
We are currently investigating the issue, and there is a direct ticket open with Facebook here:
https://developers.facebook.com/support/bugs/733783060776081/Thank you for your patience while we continue to monitor the situation.