Hi Max,
That is odd. Once logged in, go to the Admin Menu and then click the "Advanced Tools" item. Next, click the "Redirect Manager" item. Check for a redirect that is the same as the url for the home page (probably 'home'). If you see one, that's your problem so you should remove it. Just removing it will not fix the problem for anyone who has already used the site because they will have the redirect cached. Go to Administration Menu and select the "Page Manager" item. Select the Home Page and click the "Edit Settings" link. Set the url for the home page to ~/. You don't need an actual url there anyway.
If that doesn't work for you, try this:
Go to IIS and click on the site.
In the settings pane, double-click the Authentication item under IIS. Select "Anonymous Authentication" and click "Edit" on the right. Be sure the Anonymous user identity is set to a user which has read access to your web directory. I don't really think this is the problem because usually this will result in getting redirected to ~/login.aspx, which doesn't exist.
If you can't figure it out, let me know and I'll give you instructions to submit your website login information to us so we can investigate further.
Thanks,
Joe