Posting or editing a post redirects me to homepage

I’m having major issues with my WordPress installation which started today. Here is what happens:

1. I go to Write to create a new post
2. I type up a post
3. I hit Save, Publish or Save and Continue Editing
4. I get redirected to my homepage
5. No changes are saved

The same thing happens when I try to edit an existing blog post or draft. It also occurs when I try to edit my theme via the web interface or try to add/edit WordPress users.

Editing and submitting comments works normally however.

My issue was curiously similar to the one described here:

http://wordpress.org/support/topic/178990

I tried following the mod_security tip listed there but it did not help. I tried disabling mod_security via the Panel but that didn’t help either.

I also tried replacing my post.php with a fresh copy I downloaded from wordpress.org thinking that maybe it is somehow corrupted. That changed nothing.

I tried disabling wp-supecache and wp-spamfree and that did not help either.

I deleted the chache – no ball.

I explicitly excluded post.php from wp-cache hoping that has something to do with it but it did not help.

I checked and re-checked my .htaccess files many times and they seem fine. I tried commenting out the code added by WP-Cache but it didn’t help.

I upgraded my php from version 4.x to 5.x via the Panel hoping that would do something. No result.

I tried 4 different browsers (firefox, IE8, chrome and opera) on two different computers. All behave exactly the same – redirect me to the home page.

This problem started today. I haven’t made any changes to the blog other than regular posting. I was able to post normally early morning (3am EST) but when I logged in and tried posting at 4pm today this issue started.

I looked through my WordPress directory but no files seem to have changed recently. I looked through the database and I didn’t see anything out of the usual. I am at my wits end here. I have no clue what is going on.

I contacted my host to see if they can figure out what is happening here but they haven’t responded back to me yet.

Has anyone ever had a similar issue?