This happens because of the _wp_http_referer comparison in settings.php and test.php if you have a non-standard /wp-admin path (e.g. because you’re using Bedrock).
This is the problematic part of the condition:
$_POST['_wp_http_referer'] != '/wp-admin/admin.php?page=wp-reroute-email%2Fsettings.php'
A replacement that has better compatibility but preserves the intent could be:
basename( $_POST['_wp_http_referer'] ) != 'admin.php?page=wp-reroute-email%2Fsettings.php'
