AX2012 - "Calculate cash flow forecasts" ends up in a stop error "The purchase order line cannot be created because the purchase order has been finalized"

Hi,

I am using a contoso-demo-system which is also used for trainings. When I start

General ledger - Periodic - Currency requirement - Calculate cash flow forecasts

the system is working for some time and in the 1st try came back with a lot of messages that "Sales tax settlement periods" are missing. After I created them for all times (ok, only till 2014) the next try ended up in a single line in the infolog with a stop error:

The purchase order line cannot be created because the purchase order has been finalized

Now there are a lot of POs which are finalized - because of final invoicing or canceling.

How can I tell the system that these POs maybe ignored?
I am sure that I should not re-open a finalized PO :-)

Thanx for any help
Wolfram