What does this error mean? The "By User" column shows "-1 [-1]"?

How do i figure out who this user is based on this error message:

That means the user’s not logged in.


How can i tell who? I send an email to myself for every error - i am getting too many emails from this error.

How do i stop this? I am getting too many emails daily. I need to send myself emails when my apps are having issues, however why is this happening so much? Also, i cannot see what user it is - because it is “-1”: