Converting a single site to a WordPress multisite network can be a little hairy. (But it’s a LOT easier with WP-CLI.)
We recently battled a frustrating login issue after converting to multisite. We couldn’t login after enabling multisite due to a login redirect. Clicking “Login” would just refresh the page and show you the login screen.
Much of the information we found while troubleshooting had to do with config files, which we knew were correct, or the particulars of a subdomain install. We were using subfolders.
- Clear your cookies.
No seriously, that was the entire problem. WordPress was confused by pre-multisite login cookies.
This may not solve all login redirect problems with WordPress Multisite. If you are using subdomains, we highly recommend checking out Tom McFarlin’s guide, Resolving The WordPress Multisite Redirect Loop.
Let us know if this fix works for you.