Hello, we solved this on our website by makeing changes in the file:
"plugins\system\recaptcha\recaptcha.php"
We have it placed in after our custom field "address_vat", and that made this:
/** $components = array('view' => array('checkout','user'),'viewvar' => 'ctrl', 'lengthafter' => 500 , 'tdclass' => 'key', 'email' => 'data\[register\]\[email_confirm\]','password' => 'data\[register\]\[password2\]');**/
$components = array('view' => array('checkout','user'),'viewvar' => 'ctrl', 'lengthafter' => 500 , 'tdclass' => 'key', 'email' => 'data\[address\]\[address_vat\]','password' => 'data\[address\]\[address_vat\]');