The reason you get the warning now is that you are forwarding the page when it is in the middle of writing out the page (where you added the <br /> tag). Adding the <br /> tag isn't the correct solution to the issue. Did you make any modifications to the cart pages after eCart generated them? The redirect is being written incorrectly as the checkout success page instead of the cart page.
I'm not sure what the attachment is supposed to show me.