I have already solved the following problem. It was because you had to specify a page in the E-Recruiting admin area that is called after submitting the form and its URL was wrong.
In the website www.wunder-personal.de I have a problem with the form on the page “Bewerbung | Wunder Personal-Dienstleistungen GmbH”. The form is a (not Kirby-) plugin from an E-Recruiting company, which is integrated into the page with Javascript.
As soon as you send this form, the Kirby error page appears, but the sent data still reaches the E-Recruiter. What could be the reason? How can I change this so that the error page is not displayed? By the way, the Kirby debugger does not detect an error.