Just a quick message to help others who might encounter the issue.
My Wordpress was dying with the following error in ngnix error.log.
2020/04/08 13:30:18 [error] 24442#24442: *276775 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Class 'DOMImplementation' not found in /var/www/wordpress__3/wp-content/plugins/lazy-loading-responsive-images/vendor/masterminds/html5/src/HTML5/Parser/DOMTreeBuilder.php:172
After a bit of digging, it turned out that php-xml was uninstalled on my server. Not too sure why, and the apt log doesn’t mention it.
Anyway, a little sudo apt install php-xml solves the issue.