Not possible to cancel registration in PO

Hello people, 
I'm not able to unregister item in a PO. I go to Update line-->Registration. And the form is NOT opening.
The thing is, there has been a problem with the arrival journal, I suppose. The PO has 3 lines, there is a posted item arrival journal with these 3 items, but the last one does not appear as Registered in Line quantity in the PO, just stays in "Deliver reminder" field as ordered quantity.
In Inventory Transactions there are 4: registered in this PO, then only "ordered" in transfer order(Transfer order receive), then reserved physical in the same transfer(Transfer order shipment), and then reserved in SO. 
Is there a safe way to unregister the quantity or to make the Registration form work properly? 
P.S.: I manually deleted (from Marking - Transfer order) the connection between the transfer and the PO. But I cannot see what happened in the PO, because when I try to open Marking form for this item, the same happens - it doesn't open.