I do this, and it always works.
First thing in the htaccess file I insert this code
ErrorDocument 404 /404.html
there are 2 pages:
- in one page you create page 404
title browser: 404 You are lost
folder: /
filename: 404redirected.html
here you make your 404 page inserting as many stacks as you want
- then you create another one, this must be an Offsite Page
Browser Title: The Real 404
Folder: /
Filename: 404.html
in offsite options enter URL i.e. the real address of the 404redirected.html page, in your case
https://www.seminarhaus-deinsdorf.de/404redirected.html
If I explained myself badly, tell me and I’ll make you some screenshots
