4.0 fatal error, crashes entire site

Just update to 4.0 and now my whole site is down with a “fatal error” message. Here’s the stack trace from New Relic:

Stack trace
Error: Uncaught exception 'Error' with message 'Failed opening required '/nas/content/live/sevencolonial/wp-content/plugins/custom-login/vendor/composer/../opis/closure/functions.php' (include_path='.:/usr/share/pear/php:/usr/share/php')' in /nas/content/live/sevencolonial/wp-content/plugins/custom-login/vendor/composer/autoload_real.php:55
in composerRequire76b286d45d1d12ff54e52e1246a5182f called at /nas/content/live/sevencolonial/wp-content/plugins/custom-login/vendor/composer/autoload_real.php (38)
in ComposerAutoloaderInit76b286d45d1d12ff54e52e1246a5182f::getLoader called at /nas/content/live/sevencolonial/wp-content/plugins/custom-login/vendor/autoload.php (12)
in require_once called at /nas/content/live/sevencolonial/wp-content/plugins/custom-login/custom-login.php (71)
in include_once called at /nas/content/live/sevencolonial/wp-settings.php (418)
in require_once called at /nas/content/live/sevencolonial/wp-config.php (139)
in require_once called at /nas/content/live/sevencolonial/wp-load.php (50)
in require_once called at /nas/content/live/sevencolonial/wp-admin/admin.php (34)
in require_once called at /nas/content/live/sevencolonial/wp-admin/edit.php (10)