Status Code 406 Not Acceptable

Very rare types of web browsers may transmit legitimate requests for XMB pages that are incompatible with certain Apache configurations.

The server responds un-cooperatively whenever the forum's front page is requested, like this:

HTTP/1.1 406 Not Acceptable
Alternates: {"index.php" 1 {type application/x-httpd-php}}
Vary: negotiate

Since XMB does not rely on ~MultiViews, one workaround is to disable it by adding these directives to the forum's .htaccess file:

Options -MultiViews -Indexes
RewriteEngine On
RewriteBase /
RewriteRule ^$ index.php [L]
RewriteRule ^files/ files.php [L]

Note the ~RewriteBase parameter is the path portion of your forum's URL, which might be / or /forums/ including the trailing slash.