The update shows "AJAX loading error"

Why it happens

The update runs as a series of background requests. If the server blocks or cuts off one of them, the browser shows "AJAX loading error". Common causes are a security module (such as ModSecurity), a custom .htaccess that blocks PHP files, or PHP timeouts.

Fix it

1. Open your browser's developer tools (Network tab), run the update again and look at the failing request: 403 points to a security rule, 500 to a PHP error, a timeout to server limits.
2. If you changed .htaccess, make sure it lets the updater's extract.php run. The core file has the rule RewriteRule ^administrator/components/com_joomlaupdate/extract\.php$ - [L].
3. Ask your host to whitelist the update for ModSecurity, or raise max_execution_time and memory_limit.
4. As a workaround, download the update package from downloads.joomla.org and use upload and update.

Good to know

• Check the update log in administrator/logs/joomla_update.php: it shows the last step that worked.