Hi Toufiq,
I found the cause, but I don't have the skills to fix it. It's an issue with SPPB!
I made a brandnew installation for testing, including all my plugins, components (including, changings in .htacces and so on. It worked perfectly: Helix 404 page or another indiviual errror page was displayed all the time - up to that moment I set a SPPB Page as Home. From that moment on, all non existing url were redirected to home instead of Helix 404 page or another specified error page. If I set a non-SPPB page as home, helix or my own error page is displayed correctly.
If you want to test it, see hidden content of this post.
Is there any line of code I can edit to fix it? I have to work with SPPB and it's important to display an individual error page.
Thanks!