Error: Assign - 'Variant' from 'Sequence': Invalid Signature

When schedule run robot in Orchestrator, got the error message like below:

This situation happened before 1 time, and when I manually trigger in Orchestrator, no errors.
Does anyone know the root cause of this invalid signature error? Is this related to programming code or network connection or Orchestrator scheduler etc.? Kindly share your suggestions please. Thank you very much.