WordPress error 500 – Uncaught Error: Call to undefined function trailingslashit() in wp-includes/class-wp-textdomain-registry.php:103

When updating the plugins in a WordPress site prior to updating it from WordPress 6.0 to 6.1.1 the site halted with error 500. Investigating the logs I found “Uncaught Error: Call to undefined function trailingslashit() in wp-includes/class-wp-textdomain-registry.php:103“

Solution

• Download WordPress 6.1.1 from https://wordpress.org/wordpress-6.1.1.zip
• Unzip the file and replace the wp-includes folder in your site with the one from WordPress 6.1.1