I have messed up my theme-footer.php file trying to delete the kriesi.at back…

Hi Christian!

Please see private content field. Then refer to this post – http://kriesi.at/documentation/enfold/change-the-footer-text-and-link/
If you would not like to use that solution, please add following code to functions.php file of your child theme

add_filter("kriesi_backlink","new_nolink");
function new_nolink(){
$kriesi_at_backlink = "";
return $kriesi_at_backlink;
}

We recommend using a child theme – http://kriesi.at/documentation/enfold/using-a-child-theme/ as that would avoid having files that you have modified from getting overwritten when you update the theme.

Regards,
Yigit