If you get this message when you are opening the PO Module before selecting anything the default record might have the invalid data. The only way to clean this up would be using data surgery or through the backend in SQL Mgmt Studio.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .