Thanks U for your help,and favour, but these are all the things that I've done before.
I've also tried thr filtering and j_security_check for my login form.
I believe that this might be a problem of my web  server, or sth else(I really do not know!).
But at least you're the one who helped a lot.
Thanks you more and more, and waiting to hear -READ I BELIEVE!- from U more and soon.