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 Checkplease 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?