From your example for the shortcode it seems that I am supposed to enter the listID and then the groupID separated by slash.
list – MailChimp list_id or list_id/group_id if you want to subscribe to specific group.
If it supposed to mean list OR group ID then I understand there should not be any slash of course. I have tested with groupID only (no listID and no slash) and it gives the same result.. “There was a problem processing your submission”