I just installed a fresh copy of 3.0.4, went through the steps to create a multisite and everything looked good. Then, I attempted to sign in to one of the newly created Sites and the login page simply repeats itself. No errors, not even in the log. The main install works fine, this seems to be isolated just to new sites.
I've tried logging in using the Super Admin user and the Site Admin user, both do the same thing. Anyone seen this before?