I was afraid of that, but I had no idea where to look to solve it. Indeed there were some evil lines in index.php, problem solved!
Now, does anyone know how this got there? Am I safe if I change all passwords be up to date with newest wordpress? Strange thing is that this website has always been WP up to date. Other sites of mine have been “hacked” about a year after I quit updating them. They run on the same server though…