I’ve written a couple of blog posts related to multisite. But two common issues that might cause problem, I have not yet written about.

Missing to redirect the www subdomain

When you convert your WordPress installation into a multisite, you can either use subfolders or subdomains. If you decide to use the subdomain variant, each site will be matched to a subdomain. And if your installation is not using the www subdomain for the main site, this means that also the www subdomain would be considered as a separate site and you will get this error message:

Greetings Network Administrator! The network currently disallows registrations. To change or disable registration go to your Options page.

Registration has been disabled.

So after converting you installation, make sure to check the www subdomain and create a redirect in your web server configuration, if the error message shows up.

Being unable to log in to your secondary top-level domain

With both setups, using subfolders or subdomains, you can also use a different top-level domain for any of the sites. When you try this, you will most likely not be able to log in to the backend of this new site. In many online tutorials you will find a tip on what to do, but one thing is usually missing.

Setting the COOKIE_DOMAIN

The first thing you have to do, and usually find, is to set the domain that should be used for cookies. Simply add the following line to you wp-config.php file:

define( 'COOKIE_DOMAIN', '' );

This is all you need to make it work. But if you try to log in, it will most like still not work.

Solution 1: Delete the cookies

The issue with the login not working are old cookies. So you can simply delete all (WordPress) cookies in your browser and now you should be able to log in to the backend of the new site with the top-level domain.

Solution 2: Refresh the security salts

Another way to fix the issue is refreshing the security salts in your wp-config.php file with new values. You’ll find a link to a webpage to generate new values in the comment just above the constants.

This solution will treat all session/login cookies from all users on all devices as invalid – it will “log out” everyone. So while this solution will fix the issue for all users, it might cause some issues, when someone is just working on the site.

Conclusion

Converting a WordPress installation to a multisite is not too complicated. But there are some issues you should be aware of. If you haven’t given a multisite installation a try, I would highly recommend to do so, as it can be a good solution for some website needs. This blog is using a multisite to be multilingual.