problem opening a new site with forms authentication

Hi,
I have 2 sites, a normal site and administration site. The only difference in the URL is that the admin site is a subdomain (prefixed with admin) of the main site.
Now I can open each site individually and login. On the main site I have a link to the admin site that opens it via the javascript
window.open('http://admin.normalsite.com/default.aspx,'_blank');
In Chrome this works fine and opens a new page (not tab) and you can again login and do your business stuff
In IE10 a new tab is opened and the login page loaded....when the credentials are added and login pressed I get a flick and then we are back to the login page with empty fields....it appears that it attempts to go to the business page after the FormsAuthentication.SetAuthCookie(blah,false) but is not authorised and so bounces back to the login page.
The above code works fine on my development PC but not the host server with IE10??
I have to close down the ...