I have now sorted this problem.. inside admin section there is a captcha setting for the register page which I have disabled.
Maybe this will help someone one day.
Regards...

Hi jugears,

Hi, can anyone help with this error message please? I get this after clicking on "register" link.
Fatal error: ...

You're right - this happens if CAPTCHA is disabled for the contact form and enabled for the register form due to a bug in register.inc.php on line 14:

if(empty($_SESSION[$settings['session_prefix'].'user_id']) && $settings['captcha_email']>0)

... should read ...

if(empty($_SESSION[$settings['session_prefix'].'user_id']) && $settings['captcha_register']>0)

Alex