Today a site I administer started getting a 404 error when trying to access the CP at www.thedomain.com/admin.php
It was working fine and as far as I know there have been no changes to the system by the client.
A user reported getting the error 'disallowed key characters' - though I have not seen this, for me it just redirects to the site 404 page.
The only thing that is different is that they sent an mail out earlier today (batched in 100s - approx 2000) that might result in high bandwidth/traffic in response, I am not sure if that would impact it though.
Can anyone help shed some light?