I'm having this same problem. When I go to the login page and try to login, I can't login, but if I try again it says I'm already logged in... then I have to click on the CMS link in the bottom right to actually get to the CMS. I also can't save or publish anything to the published site, it's like an endless saving loop.
The CMS loaded fine. My custom template took and I entered content. Then one day (the day I was going to show the site to my client, natch), the /admin page went wonky.
It says, "Enter your email address and password to access the CMS," but the form fields have vanished, so it's impossible to login. Same thing at /Security/login.
Sometimes in IE6(!) I'll see the form and be able to log in. But never in Firefox. I work on a PC, my client uses a Mac. He hasn't been able to edit any of his content. I have to do it for him (when I can locate a login form).
I'm more of a front-end (xhtml / css) guy with some (very) basic understanding of the back-end. Anyone have any ideas?
If you go to /admin and it gives a 404 then you should make sure a) a cms folder exists and there is a cms/_config.php with the correct director rule - something like 'admin/$Action/$ID/$OtherID' => 'CMSMain', should be in the list