Some users have reported problems loading the list on the Tag Groups admin page. In most cases, this is caused by another plugin. The other plugin causes your site to output a warning or alert that interferes with the data transfer to your browser.
Try disabling other plugins one-by-one #
Try disabling other plugins one-by-one.
If you can see the JSON data, that may give you a hint about the plugin causing trouble. Check any text before the opening curly brace. For example: Warning: xyz in /some-plugin/index.php{"data":"success" ...
Try other steps #
Go to the “Tag Groups”, then “Tools”, then “System Information”. Check if the “Ajax Test” option shows a green check mark. If you see a red X, this post has more information where to search.
Alternatively, you can open the JavaScript Console in your browser on the Tag Groups Administration page. Reload the page and check the error or log messages for anything that gives a hint about the cause.
A final suggestion is to add the following lines to your wp-config.php file. Then reload the Tag Groups Administration page and check the content of the debug.log file in the /wp-content/ folder so see if there is any error message.
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );