url with .html extension
Okay. I found the solution If anyone runs into the same issue.
This must be a server problem which I can't solve so there is a htaccess solution to remove the .html extension:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)\.html$ /$1 [L,R=301]