You may check the wp-content/siteground-migrator.log for the issue.
One reason for the critical error is caused by Googles Site Kit Plugin (probably since 1.2.0.) so you need to delete it first.
From the log: [message] => Uncaught PharException: tar-based phar “/[…]/public_html/wp-content/uploads/[…]/plugins/google-site-kit.tar” cannot be created, filename “dist/assets/js/chunk-googlesitekit-adminbar~chunk-googlesitekit-setup-wizard~chunk-googlesitekit-setup-wizard-proxy[…].js” is too long for tar file format in /[…]/public_html/wp-content/plugins/siteground-migrator/includes/class-siteground-migrator-files-service.php:205
