Hello,
Same problem:
PHP Fatal error: Uncaught Error: Class 'MetaBox\Columns\Processor' not found in /var/www/html/wp-content/plugins/MY-PLUGIN/vendor/meta-box/meta-box-columns/meta-box-columns.php:30
I use composer https://github.com/wpmetabox/library/blob/master/composer/composer.json and all plugins are updated
