"Request Error" in History & Monitoring tab
What to do when the Following server error message appears in
You see this error because in your “JungleMail Jobs” list there is a job (or jobs) in which the CreatedBy column refers to an invalid user. Perhaps that user has been deleted from the User Information List on that site collection. And if you click on this user at documents you will receive the message: “System.ArgumentException: User cannot be found.”
Find and delete the JungleMail jobs items in "JungleMail Jobs" list created by that invalid user.