This error message is related to the default Magento or third-party captcha. When you use a custom theme, it hides "I'm not a robot" checkbox and it's not available for submitting. Therefore, ''Incorrect CAPTCHA'' message is displayed.
The solution is pretty easy: just disable the default Magento or third-party captcha. The ''Incorrect CAPTCHA'' message should be gone. If it still appears, please do not hesitate to create a support ticket.