Cannot Edit or Delete a Post

I am having extreme difficulty, for some reason today I cannot write a new post, edit a post or delete a post.

I am currently using 2.1.3 but I have tried deleting that and installing 2.0 to no avail.

I have deleted the database and restored from a 2 month old backup, to no avail.

Swapped themes to classic, deactivated and deleted all plugins, rebooted many times, tried in IE as well as FF, all to no avail.

I can write or edit a post so long as I do not enter any html in code view and if I do not make text bold etc in the visual view. So basically I cannot enter anything other than plain text.

If I try and enter html or bold text then when I click save or publish I am immediately taken to the home page, and the post is not saved.

Also, if I try to delete a post, it correctly comes up with the warning dialogue box, but when I click OK, again I am taken directly to the home page and the post is not deleted.

I am at my witts end. Any help would be much appreciated.

Thanks in advance.