A loopback is when your own server or website tries to connect to it self.

WordPress uses this functionality to trigger scheduled posts, and other scheduled events that plugins or themes may introduce.

They are also used when making changes in the Plugin- or Theme-editor, by connecting back to the website and making sure that the changes made does not break your website.

Troubleshooting loopback issues

If you are having problems with scheduled posts or other timed events not running, or seeing Site Health warnings about loopbacks failing, you may want to troubleshoot these.

The most common cause of loopback failures is a plugin or theme conflict, you should start by following the normal troubleshooting steps:

Common troubleshooting steps

• Deactivating all plugins (yes, all) to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s). If you can’t get into your admin dashboard, try resetting the plugins folder by SFTP/FTP or phpMyAdmin (read How to deactivate all plugins when you can’t log in to wp-admin if you need help). Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old.
• Switching to a Twenty-Something theme to rule out any theme-specific problems. If you can’t log in to change themes, you can remove the theme folders via SFTP/FTP so the only one is twentytwentythree. That will force your site to use it.
• If you can install plugins, install the plugin Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.