well I use the option of mail and message both in my setup and have never seen an issue. The problem is because your sender userid or product field is empty.
As I see it since the email your referring to is only used by the cron tasks either through the footer cron.php or the server sendnotice.php script which ever your using does not matter but we grab the admin userid based off your admin email when your using the messaging system. We have a function that even does it look here.
Content visible to verified customers only.
Based on what I am seeing your entering an admin email for the system that does not actually correspond to the admin on the site so no userid can be returned.