CSPCSP_PROCESS_ORDER_ERRORSNOM_MSGValidation failed for the field - Bill To

This happened all of a sudden. It's still unclear whether this is due to setup or some technical cause.

Situation:

Enter a Parts Requirement with ship-to Technician.
Enter item with a manual entered source, and choose the inventory_org from a different OU.
Press apply: error "CSPCSP_PROCESS_ORDER_ERRORSNOM_MSGValidation failed for the field - Bill To"

Yesterday morning it was working fine, yesterday afternoon it wasn't.

Taking another resource (existing) and linking subinvs to it (which it didn't have yet), and using that engineer also doesn't work.

Ship-to customer still works, ship-to engineer within same Inv.Org still works (creates reservation).