This solution worked for me. It's well worth the $12. My users are now happy again.
I was running 1.8.0 when this happened about 24 hours ago.
Update: 12/20/15 - Even though our users are getting email notifications, and if they click on the Messages button/icon they will see their messages. However, the Messages button/icon is not being updated to show how many Messages. For example, if doesn't update to say to have 1, 2, etc Messages. So, I sill need some heop.
With this plugin the problem disappearshttp://www.oxwall.org/store/item/688