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.
I narrowed my problem down to that it sounded like I was meant to do some site configuration using the Domain Mapping plugin. The problem then was, that I did not have Super Admin->Domain Mapping as an option in the Network Admin area (nor could I see any other options that looked relevant either).
The plugin description page talks about updating the database. So I had a look there, and eventually I found the wp_domain_mapping table. I had to add a new record to this table with the blog_id and domain of the new website, and now I can log in to the admin area for that site okay.
I have now also found out the way you are meant to do it. For the super user, when logged into the admin area, you can go to Tools > Domains to configure the domains and blog_id mappings. For me, this is in the main dashboard for that user’s website, not in the Network Admin dashboard. This is rather confusing, but may be due to how I have my website set up.