Hi there,
Can you check the variant you have in the background process? If your manual run produces results in the ZL but your background process doesn't, then I suspect it is because there is a difference between the 2.
Secondly, is your payroll area locked when you run the background job?
Finally, I suggest you pick an employee where the background job is failing and run that variant for that individual, with the log on, in the background. Then you have results to compare when you run it manually.
Let me know if this helps...
Will