[Krisstoffer]

mod works fine here with nginx, only change needed is of course the instructions for apache won't work for nginx but they are simple enough:

error_page    400 /forum_400.php;
error_page    401 /forum_401.php;
error_page    403 /forum_403.php;
error_page    404 /forum_404.php;
error_page    500 /forum_500.php;

You could also go along the lines of:

error_page    500 501 502 503 504 /forum_500.php;