Hello Leo,

I checked this issue and found that you have disabled Received System Emails for admin user that's why this error is coming.
I have enabled it and send contact us and it's showing email were sent to admin. I've disabled it again.
Let me know you receive email or not.

Payplans contact us email will send email to admin user with Received System Emails set to yes.
If you don't want to enable for admin user's then let me know i'll fix it at your end.
Update me.