FB.ui with method 'feed' is resulting in an 'unknown error' and CAPTCHA request

We're using code that we've used before, so I suspect that this may be site-related. In using a standard:

FB.ui(
  {
    method: 'feed',
    app_id: '<?= $LDP->config->facebook->id ?>',
    name: 'Post Name',
    link: flink,
    picture: "https://www.domain.ca/templates/visual/images/share.gif",
    caption: "Caption",
    description: 'Join the fun today!',
    actions: [
      { name: "Check it out!", link: flink }
    ]
  },
  function(response) {
    if (response && response.post_id) {
      alert('Post was published.');
    } else {
      alert('Post was not published.');
    }
  }
);

It first displays the expected share dialog, and when you click the button at bottom right to go through with the stream publication, a new popup appears with:

Title: Require Captcha

unknown error

Security Check please enter the text below

[captcha appears]

The only button is "Ok". Correctly solving the Captcha results in a crash (Facebook servers throwing a 500 error).

Any ideas?

Solution

Turns out this is a bug with Facebook (broken captcha issue). The pop-up is an innate anti-spam system, but people should be able to succeed with the CAPTCHA. I'd filed a private bug with Facebook, and it's slated to be fixed apparently.