Unable to reset password via recovery mode

I've recently inherited a former colleague's MacBook Pro. Unfortunately the machine is password protected and therefore inaccessible. I've tried to reset the password via the recovery mode. After entering the new password, it gives me an error message "Authentication server could not be contacted". And that's as far as I can go. Please could someone explain to me what this message means and perhaps suggest a way I might be able to overcome this obstacle.

Many thanks,