Geesh, this bug has been going on so long that it’s really a bit of a disappointement that nobody seems to have nailed it.
The problem in PHP5 seems to have something to do with the redirects (at least in my case). Every place where WP tried to set the header to redirect the browser is where . . . it doesn’t work. That’s why none of the login stuff works for anybody. You login, you hit the login script which auth’s you and then it tries to redirect you to the admin page. Go check out anyplace where WP tries to set the header and it breaks. Funny thing though, if you telnet to port 80 and request the same page, the redirect works!!!! Now, one of the only logical reasons for this to happen is if something else already sent a header and this second header is being ignored by the browser (thus the blank screen instead of an error page).
I would have fixed this and posted the code but the last time I attempted to point out problems in the WP code I was told that if I don’t like the ways things are I should just go build my own.