ME21N: Error "Enter cost center" when using Account Assignment Category A

Dear gurus,

we would similar to usage Account Assignment Category A Asset successful a PO.

In that Category tract Cost Center is NOT acceptable arsenic mandatory successful customizing.

Still, erstwhile I participate an plus fig and property return, I get an error:

Enter Cost Center
Message No. ME083

Even much confusing: When I support a valid outgo halfway fig successful tract Cost Center, the mistake inactive is displayed.

Any ideas?

Thanks
Jens