trigger <str> forced an error

  Explanation:  The specified trigger executed an  action  defined
  to produce this exception.

  User Action:   This  may  be  an  expected  occurrence  in  some
  situations,  but in general determine the conditions that led to
  the update that caused  the  triggered  error,  eliminate  those
  conditions  (it may be necessary to discuss a solution with your
  database manager), and try the operation again.