Maybe the download of the file got corrupted?
Here's the right one:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^change/email/([0-9]+)/?$ profile.php?action=change_email&id=$1 [L,NC]
RewriteRule ^change/email/([0-9]+)/([a-zA-Z0-9]+)/?$ profile.php?action=change_email&id=$1&key=$2 [L,NC]
RewriteRule ^change/password/([0-9]+)/?$ profile.php?action=change_pass&id=$1 [L,NC]
RewriteRule ^change/password/([0-9]+)/([a-zA-Z0-9]+)/?$ profile.php?action=change_pass&id=$1&key=$2 [L,NC]
RewriteRule ^delete/([0-9]+)/?$ delete.php?id=$1 [L,NC]
RewriteRule ^delete/avatar/([0-9]+)/?$ profile.php?action=delete_avatar&id=$1 [L,NC]
RewriteRule ^edit/([0-9]+)/?$ edit.php?id=$1 [L,NC]
RewriteRule ^email/([0-9]+)/?$ misc.php?email=$1 [L,NC]
RewriteRule ^forum/([0-9]+)/?$ viewforum.php?id=$1 [L,NC]
RewriteRule ^forum/([0-9]+)/page/([0-9]+)/?$ viewforum.php?id=$1&p=$2 [L,NC]
RewriteRule ^forum/([0-9]+)/rss/?$ extern.php?action=feed&fid=$1&type=rss [L,NC]
RewriteRule ^forum/([0-9]+)/atom/?$ extern.php?action=feed&fid=$1&type=atom [L,NC]
RewriteRule ^help/?$ help.php [L,NC]
RewriteRule ^login/?$ login.php [L,NC]
RewriteRule ^logout/([0-9]+)/?$ login.php?action=out&id=$1 [L,NC]
RewriteRule ^mark/read/?$ misc.php?action=markread [L,NC]
RewriteRule ^new/topic/([0-9]+)/?$ post.php?fid=$1 [L,NC]
RewriteRule ^new/reply/([0-9]+)/?$ post.php?tid=$1 [L,NC]
RewriteRule ^new/reply/([0-9]+)/quote/([0-9]+)/?$ post.php?tid=$1&qid=$2 [L,NC]
RewriteRule ^post/([0-9]+)/?$ viewtopic.php?pid=$1 [L,NC]
RewriteRule ^post/([0-9]+)/?\#p([0-9]+)$ viewtopic.php?pid=$1#p$2 [L,NC]
RewriteRule ^register/?$ register.php [L,NC]
RewriteRule ^report/([0-9]+)/?$ misc.php?report=$1 [L,NC]
RewriteRule ^request/password/?$ login.php?action=forget [L,NC]
RewriteRule ^rules/?$ misc.php?action=rules [L,NC]
RewriteRule ^search/?$ search.php [L,NC]
RewriteRule ^search/([0-9]+)/?$ search.php?search_id=$1 [L,NC]
RewriteRule ^search/([0-9]+)/page/([0-9]+)/?$ search.php?search_id=$1&p=$2 [L,NC]
RewriteRule ^search/new/?$ search.php?action=show_new [L,NC]
RewriteRule ^search/recent/?$ search.php?action=show_24h [L,NC]
RewriteRule ^search/unanswered/?$ search.php?action=show_unanswered [L,NC]
RewriteRule ^search/subscriptions/?$ search.php?action=show_subscriptions [L,NC]
RewriteRule ^search/user/([0-9]+)/?$ search.php?action=show_user&user_id=$1 [L,NC]
RewriteRule ^subscribe/([0-9]+)/?$ misc.php?subscribe=$1 [L,NC]
RewriteRule ^topic/([0-9]+)/?$ viewtopic.php?id=$1 [L,NC]
RewriteRule ^topic/([0-9]+)/page/([0-9]+)/?$ viewtopic.php?id=$1&p=$2 [L,NC]
RewriteRule ^topic/([0-9]+)/new/posts/?$ viewtopic.php?id=$1&action=new [L,NC]
RewriteRule ^topic/([0-9]+)/last/post/?$ viewtopic.php?id=$1&action=last [L,NC]
RewriteRule ^topic/([0-9]+)/rss/?$ extern.php?action=feed&tid=$1&type=rss [L,NC]
RewriteRule ^topic/([0-9]+)/atom/?$ extern.php?action=feed&tid=$1&type=atom [L,NC]
RewriteRule ^unsubscribe/([0-9]+)/?$ misc.php?unsubscribe=$1 [L,NC]
RewriteRule ^upload/avatar/([0-9]+)/?$ profile.php?action=upload_avatar&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/?$ profile.php?id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/identity/?$ profile.php?section=identity&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/setup/?$ profile.php?section=setup&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/messaging/?$ profile.php?section=messaging&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/signature/?$ profile.php?section=signature&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/avatar/?$ profile.php?section=avatar&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/display/?$ profile.php?section=display&id=$1 [L,NC]
RewriteRule ^user/([0-9]+)/admin/?$ profile.php?section=admin&id=$1 [L,NC]
RewriteRule ^users/?$ userlist.php [L,NC]
RewriteRule ^users/([0-9\-]+)/([a-z_]+)/([a-zA-Z]+)/(.*?)/page/([0-9]+)/?$ userlist.php?show_group=$1&sort_by=$2&sort_dir=$3&username=$4&p=$5 [L,NC]
</IfModule>
EDIT: I see the update added some extras for the admin inteface:
Isolated the admin interface in it's own folder and added the admin scripts to the SEF URL scheme.
Maybe that's giving a problem with you; the code I posted above is still quite old but works