Thanks for this hint! It was the following code in ready.php:
$ this-> addHookAfter ('Pages :: trash', null, function ($ event) {
if ($ this-> wire ('user') -> hasRole ("editor")) $ this-> wire ('session') -> redirect ("/");
});
With this code we want to prevent users from being directed to the treelist after deleting a page. The problem was, that the role editor was also assigned to the superuser.
I am a processwire beginner and therefore I appreciate the support of this forum!