I just ran into a similar problem that returned the same error code. I found that the FileOrder SP returns this error when you try to Complete an order that is setup with resultable items. In my case, it didn't make sense that the order had associated resultables - I was auto-completing follow-up orders - but in your case the order may be resulted. If not, then remove the resultable items from the OID. Other options are to complete the order by assigning a result value via a FileResult call; or you could create two instances of the same order - one that can be resulted and one that can be completed via your interface. There are certain statuses that an order can be updated with, so maybe a status other than Complete would work for you too.
I hope that helps.