thanks martimiz, i did it as you said and started a test-installation from scratch and re-created my code. it turns out that the issue is not related to the contact-page but to some code i inserted to created automatic short-urls in the page class… yay.

well at least the issue is resolved even if i got a new thing to check. ;)

btw using the return-block in the submit-handler mentioned before works just fine. it's just overriding the variables used in the regular contact-form.
i actually like that behavior, as it enables you to show a proper feedback and prevent a user from filling out and submitting the form once again (without a simple reload).