wp-admin login – Error 406 Not Permitted

Just over the past 4 – 5 days, more and more of my wordpress sites have started to have the same problem.

With the main admin account being ‘admin’, and a fairly strong password, the sites have secome to the following:

when you browse <install-directory>/wp-admin, you get shown the normal wordpress login page, after entering in correct admin username and password, the site with show a http: 406 Not Acceptable apage in IE and Chrome OR the words “Not Permitted” (nothing else, just the text)

I’m unable to access the dashboard and admin anything, what can id o ?