PHP Fatal error: Uncaught Error: Call to undefined method WP_Textdomain_Registry::reset() in /home/XXXXX/public_html/wp-content/plugins/wpml-string-translation/classes/MO/Hooks/LanguageSwitch.php:139

wpml.org/forums/topic/the-wpml-string-translation-giving-error/

You need to update your WPML.
Somehow the site lost the WPML key.

Add it on your-domain.com/wp-admin/plugin-install.php?tab=commercial

https://github.com/qTranslate/qtranslate-xt/