Does the error message show a job name or a long string of characters that is the ID of the job? If it is the latter then it may be that you have made changes to the job since it was scheduled.

When you schedule a job using Enterprise Manager it uses the ID of the job to identify it. If you then make a change to the job and save it, even without changing the name of the job, it will assign a new ID to that job.

Try either rescheduling the job or changing the schedule to use the job name instead of the ID.