Disappearing comments

Our wordpress blog suddenly doesn’t allow comments. Some of the comments posted have disappeared, and some other return a comment that says

You don’t have permission to access /cgi-bin/donothing.cgi/www.cyber-langues.asso.fr/blog/wp-comments-post.php on this server.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.