Status Code 406 Not Acceptable
From XMBdocs
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.