Unsubscribe Not Working

I am helping a friend. She is using this plugin, and tells me the Unsubscribe link is not working.

This is the Unsubscribe link from the end of her newsletter:

http://clarityforyou.com/?na=u&nk=1-78eb03e755

if clicked, http://clarityforyou.com/newsletter/?nm=unsubscription&nk=1-78eb03e755 appears in the address bar, and she gets a 404 error and “Ooops, page not found” message.

Any idea how to resolve this?

https://wordpress.org/plugins/newsletter/