I had the same issue using EHP6 SP007 and the solution was to apply the instructions from SAP Note 1904908...
Resolution
Remove "X" in field POHEADERX-DIFF_INV / POHEADERX-SUPPL_VEND if you don't want system take "invoicing party" / "goods supplier" from POHEADER-DIFF_INV / POHEADER-SUPPL_VEND.
Best Regards