Mandatory parameters are missing
Issue #2726209 on drupal.org by podarok.
When there is an entity_browser and devel installed - no way to load browsers list when devel is enabled because of
Uncaught PHP Exception Symfony\\Component\\Routing\\Exception\\MissingMandatoryParametersException: "Some mandatory parameters are missing ("machine_name", "step") to generate a URL for route "entity.entity_browser.devel_load"." at /var/www/docroot/core/lib/Drupal/Core/Routing/UrlGenerator.php line 171, referer: http://drupal.192.168.56.132.xip.io/admin/config/content/embed/button/manage/block
Disabling devel helps