10000673 
Message
In "Handled by" dropdown list, select valid user
Diagnosis
From the SAP Business One Main Menu, you have chosen
Service 
Service Call 
.In the Service Call window, on the General tab, in the Handled by dropdown list, you have selected a user who does not exist in SAP Business One.
Procedure
In the Service Call window, on the General tab, in the Handled by dropdown list, select a valid user.