WordPress MU

When I post a comment in any blog I get the following:

Error 404 - Not Found

On this url: htt://xxxxx/wp-comments-post/

I have viewing .htaccess file but I dont understand what's the mistake.

Help, please.

give this thread a look.
http://mu.wordpress.org/forums/topic.php?id=863
Initially the thread starts off about images but it also (eventually) covers this 404 comments problem (or a 404 comments problem that is similar)

Hope this helps