I added a RewriteRule to make the path of '/some-page' opening the '/some-page/index.php' but showing '/some-page/' in the browser's address bar. This works fine as expected.

On the other hand I have now the problem that whenever I type a not-existing path like e.g. '/this-does-not-exist.html' I am prompted a 500 message and not redirected to the 404 page anymore.

And if I type e.g. '/kontakt' I am prompted with an 404. But if I type '/kontakt.php' it works. It should show '/kontakt.php' if I type '/kontakt' and should show '/kontakt' in the browser's address bar.

Here is the part of my .htacces:

ErrorDocument 400 /400.php
ErrorDocument 401 /401.php
ErrorDocument 403 /403.php
ErrorDocument 404 /404.php
ErrorDocument 500 /500.php
ErrorDocument 503 /503.php

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ $1.php [L]