sorry to butt in on this conversation - and i am new here. but i am also seeing similar issues (with a few twists) and am trying to get a resolution.
in my case i also get the 404 on the login from the regular dnn site.
if i try and log in to the admin side of things using /?ctl=login added to my url, i get to the admin login, i log in, and then immediately there are pop up dialog windows saying welcome to your site, sign up for a newsletter, etc, and they will not go away. it's like the page is in an indefinite redirect loop to itself and keeps popping up the windows. i cannot get any further not even to change my superuser information.
strangely, when i went to the dnnsoftware site itself to check out the forums, i forgot what my password was (been a while since i was there) and i requested a reset. when i click on the reset link they send me i just get redirected immediately to their home page and i cannot even reset my password. i tried this several times in 4 different browsers just to make sure...