Compatibility with previous versions of Captcha

Hmmm, did you try turning off/updating the menu cache?

Also be sure to have at least Captcha 2.1.

Get Captcha 2.1. I'll adapt reCAPTCHA to use the new (?) Captcha API once the next release of it officially comes out.

Captcha 2.1 seems to be the latest release (both from the development snapshots and official releases. I'll put a note in the readme that 2.1 is required.

... Please let me know if this fixes the problem. Worst comes to worst, I can implement the old Captcha API ontop of what's in there already.

You definitely need to use the latest version of Captcha, I couldn't get reCaptcha to work without it. However, the latest version of Captcha is buggy as hell, so I reverted back until the outstanding issues are resolved.

I'm switching this issue to become a request for compatibility with Captcha versions earlier than version 2.1.

I'm not going to take on this task. I'm going to, rather, work on fixing the bugs in the current version of the Captcha module. If anyone gets it working with the previous Captcha API, then please feel free to post up a patch.

The backport I just posted for Drupal 4.7.6 uses the old captcha API. In case someone wants to use it for 5.1, I think that dropping the following two hooks in the 5.1 version should do the trick, but I don't have a 5.1 setup to test this myself:

/**
 * reCAPTCHA implementation of hook_captchachallenge
 */
function recaptcha_captchachallenge(&$form) {  
  global $recaptcha_api_server, $recaptcha_api_secure_server, $recaptcha_verify_server;
  @(include_once('recaptcha/recaptchalib.php')) or _recaptcha_library_not_found();
  drupal_add_css(drupal_get_path('module', 'recaptcha') . '/recaptcha.css');
  $recaptcha_api_server = variable_get('recaptcha_api_server',
    isset($recaptcha_api_server) ? $recaptcha_api_server : 'http://api.recaptcha.net');
  $recaptcha_api_secure_server = variable_get('recaptcha_api_secure_server',
    isset($recaptcha_api_secure_server) ? $recaptcha_api_secure_server : 'https://api-secure.recaptcha.net');
  $recaptcha_verify_server = variable_get('recaptcha_verify_server',
    isset($recaptcha_verify_server) ? $recaptcha_verify_server : 'api-verify.recaptcha.net');
        
  $form['captcha_response'] = array (
    '#type' => 'item',
    '#description' => recaptcha_get_html(variable_get('recaptcha_public_key', ''), 
      $_SESSION['recaptcha_error'],
      variable_get('recaptcha_secure_connection', FALSE)),
    '#weight' => 0,
    '#required' => TRUE,
    );
  return $form;
} // function recaptcha_captchachallenge

/**
 * reCAPTCHA implementation of hook_captchavalidate
 */
function recaptcha_captchavalidate(&$captcha_word, &$correct) {
  @(include_once('recaptcha/recaptchalib.php')) or _recaptcha_library_not_found();
  if ($_SESSION['captcha'] != '') {
    $resp = recaptcha_check_answer(
      variable_get('recaptcha_private_key', ''),
      $_SERVER['REMOTE_ADDR'],
      $_POST['recaptcha_challenge_field'],
      $_POST['recaptcha_response_field']
    );
    if($resp->is_valid) {
      unset($_SESSION['recaptcha_error']);
      $correct = TRUE;
    }
    else {
      $_SESSION['recaptcha_error'] = $resp->error;
      form_set_error('captcha_response', t('The reCAPTCHA code you entered was incorrect.'));
      $correct = FALSE;
    }
  }
}

Just a note that a new version of Captcha was just released and is much more stable then what it was before.

I'm setting this to fixed, as Captcha 3.x is WAY more stable, and a Drupal 4.7 branch is now around.