Cannot post

Hi, when I try to publish a new post I’m redirected to the homepage of my website, instead of the usual WP page containing the success/error message. The post is not published and I cannot even modify the old ones. Another symptom is that, once I wrote a post, I can’t see it in the preview: there’s only the title, but the post is blank.
This happened with WP 2.3, apparently withour reason (I didn’t change anything). I updated to 2.5 but the problem still remains, even when I disable all the plugins.

Any suggestion?

Thank you 🙂