I setup WordPress Multisite a while ago. I added one on a custom domain, and more on subdomains of the main site. Since then I've added more on subdomains, and so far everything has worked fine.
I added another on a custom domain, and for some reason I cannot log into it. Each login attempt redirects to the login page again. All the other existing domains still work just fine.
What I Tried
I'm pretty sure it is cookie related. I was able to login once by resetting the password, but then it didn't login again. I've tried multiple browsers to see if that was the issue.
I checked the .htaccess to see if there were special directives for either of the custom domains, and didn't find anything. I compared the settings for both sites (from the Network Admin dashboard) and didn't see anything out of the ordinary.