Hello
The planned order is not firmed by the exeption message. What happens is exactly the opposite, the exception message is triggered because the planned order is firmed.
I still think that the planned order is firmed because of the action control. Somehow, function module MD_SET_ACTION_PLAF is called and the action control is executed, scheduling and firming the order.
If you did not find anything with transaction CODE_SCANNER, maybe it's an RFC or something else running the planned order scheduling. Maybe it's APO or another external scheduling system.
At last, did you check the user who changed the planned order on table PLAF, as already suggested? He must know which changes were carried out.
Best regards
Caetano Almeida