When upgrading drupal 8.6.15 the Updating twig/twig (v1.39.1 => v1.40.1) broke my site with:
The website encountered an unexpected error. Please try again later.
To see the cause of the error had to add this to web/site/default/settings.php
$config['system.logging']['error_level'] = 'verbose';
Then I can see the following error on the web site:
Uncaught PHP Exception Twig\Error\SyntaxError: "An exception has been thrown during the compilation of a template ("Attribute "name" does not exist for Node "Twig\Node\CheckToStringNode".") in "core/themes/bartik/templates/block--system-menu-block.html.tw
Solution was to downgrade twig
$ composer require twig/twig:1.39.1