404 when posting formatting or HTML

Good morning, afternoon or evening!

I seem to have a rather strange problem. This problem developed without me changing anything on the site at all.

Since the problem has developed I have:
– Upgraded to 3.6
– Clean install of WordPress
– Cleared cache
– Deactivated all plugins
– Switched to the Twenty Twelve theme
– Re-saved Permalink Settings
– Applied a number of ‘fixes’ to .htaccess
– Checked ‘General Settings’ are correct
– Removed any ‘Update Services’ in ‘Writing Settings’

Now, onto the problem.

If I write a post with no formatting or HTML it will post just fine.

If I add any formatting or a single bit of HTML (be it Bold, Underline… anything!) it will not post and will return a WordPress 404 error.

When editing the post the address is https://diverse-events.com/wp-admin/post.php?post=300&action=edit

When pressing ‘update’ it displays the 404 error page with the address bar showing: https://diverse-events.com/wp-admin/post.php

I appreciate any help ??