I am sorry, but I do not see what you see.
I reopened all your screenshots, but I do not see anywhere the message User has no authorization for plant 1204
I see further no evidence that PO no.4500000511 created under 1204
I have not seen any evidence that MANAS_AVAALI has SAP_ALL and SAP_NEW
I cannot much say about SAP_ALL and SAP_NEW, I never had it in my career, not even in a IDES system, and our firefighters and superusers don't have it either. So I cannot tell if this can be overruled by any other setting. That's the reason I moved this discussion to Security.
I can just give wild guesses in regard to archiving. I would check if the exits are used for purchase order archiving where you can do extra checks to exclude POs from being archived. Maybe the wrong error message is triggered in such case.