Unable to publish custom story with mention tagging

Question

My app was doing well yesterday and posting custom stories with mention tagging but from today its not working and telling me : -

Received Facebook error response of type OAuthException: Unknown path components: /mynamespace:customaction

Error code : 2500

{
  "error": {
             "message": "Unknown path components: /namespace:customaction", 
             "type": "OAuthException", 
             "code": 2500
           }
}

$response = $facebook->api(
  'me/mynamespace:customaction',
  'POST',
  array(
    'treat' => "http://www.mywebsite.com/12321425125"
  )
);

I have not changed any configurations or anything and it has stopped working all of a sudden. Please help me identify the issue - maybe there is an issue at Facebook end.