Just thought I’d post this, more for my own future reference than anything else. I was trying to set up a new blog on my wordpress multisite installation that uses the domain mapping plugin. I set up the new site, but when I tried to log in to the admin area, I was just redirected back to the login screen.
I tried all sorts of different things, often involving clearing cookies after each attempt failed. Now, the reason I had this problem is probably different to the reason other people seem to have this problem. If you’re having the same problem, you can find a thread on the wordpress forums with various different solutions you can try here: [resolved] [closed] Redirected to login screen after Multisite install.