"Page Not Found"

I am facing the same problem after upgrade to WordPress 3.5.2. I never faced this problem before. Hitting “Save draft” or “Publish” takes me to a 404 error page and the URL is <site-url>/wp-admin/post.php.

It is a bit strange. Following are my observations:

1. I am able to update some old posts but not some others.

2. If I create a new post with empty body (or 1-2 paragraphs), I am able to save or publish it.

3. If I create a large new post with body of over 1000 words (a rough calculation), I again hit this issue.

4. A very serious issue is if I hit this problem by clicking “Publish”, it also makes my site inaccessible from there on, as if my IP has been hotlisted and access has been denied. All other sites remain accessible (including mine from a proxy server). If I restart my modem or change my IP in any way, my site becomes accessible again. This behavior is not seen if I hit the problem after clicking “Save draft”, in which case only the 404 error is seen but my site remains accessible.

This is an extremely serious problem and has rendered my website literally unusable as I cannot publish new blog posts or update old ones. Kindly fix this at the earliest!

I have been hosting my blog on WordPress for the last 3-4 years and have never faced this problem. I have also not added any new plugins recently. I am pretty sure this problem started just after the WordPress 3.5.2 upgrade itself.

Just to update, after disabling mod_security for my site the issue went away. For those who are interested, the problem is described well here: http://www.binarymoon.co.uk/2011/05/wordpress-mod-security-2/