When SSL is enabled, the communication with the captcha server is still in normal mode, resulting in warnings from the browser.
The problem is in /lib/form/recaptcha.php and the routines using this lib.
On line 88 the function call: return $html . recaptcha_get_html($CFG->recaptchapublickey, $error);
is not using the 3rd parameter (true/false) for enabling the secure captcha serve call.

temp patch for systems using ssl is to force the secure server call by changing line 88 to:
return $html . recaptcha_get_html($CFG->recaptchapublickey, $error,true);