{def $attribute=$content_object.data_map.captcha}
instead of
{def $attribute=$content_object.data_map.anti_spam}
Now the captcha image is displayed. whenever the validation does not work proper. The feddback form is always sent even if the captcha user input is wrong
now I have reactivated the recaptcha extension. It seems to work fine. When I tried first, I had the problem, that the form was sent even if the entered captcha text was incorrect.
I think I made a mistake by implement in my template.