Running WordPress MU I activate "Advanced Export for WP & WPMU" and click the "Advanced Export" page and get "Cannot load ra_export". I tracked down the error its triggering in /wp-admin/admin.php and found it's looking for the file /path/to/site/wp-content/plugins/ra_export. Not only does it lack the .php extension, but it's an underscore instead of a hyphen (the plugin fime is ra-export.php).
I can't figure out if there's a bug in MU, the plugin, or something is wrong in mod_rewrite or my .htaccess file. Any ideas?