Facebook has confirmed that the problem with sending replies to private messages is now resolved and you should no longer see any errors when doing this action in Sprout. Please contact firstname.lastname@example.org if you experience any continued issues.
Sprout is currently experiencing issues with replying to Private messages from Facebook in Sprout Inbox, and our engineers are investigating the root cause of the problem. Please subscribe to receive additional updates. We apologize for the inconvenience.