Form state redirects do not respect https
On another server the same is not working: on this server the domain
  is only reachable via https - not via http


Perhaps a redirect to force HTTPS was not set up on your other server.

see Enabling HTTP Secure (HTTPS). You can do this by altering the VirtualHost definitions on your 2nd server: 


&lt;VirtualHost *:80&gt;
    ServerName www.example.com
    Redirect "/" "https://www.example.com/"
&lt;/VirtualHost&gt;



or alternatively you can do it in the .htaccess file by adding:


RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]