Jboss 404 while restarting instead of custom error page

I'm running an Apache2 Webserver and a Jboss 6 behind it connected with mod_proxy.

When I stop the JBoss and open the website in the browser I get my custom 404 error page.

When I restart the JBoss again i get the default JBoss 404 Page instead (Only while the server is starting).

I set my error page with :

ErrorDocument 404 /errordocument.html

Why does Apache not show my custom error page while JBoss is starting? And how can i change it?