There seems to be an error in OrderLine_GetDataArray. I experience that this function does not return any orderlines even though there is in fact a lines on the order. I have confirmed that I parse orderline-handles to the OrderLine_GetDataArray-function and I can see that there is in fact a line on the order by going to the admin-system. I use Order_GetLines to fetch handles for all lines on the order. The single orderline-handle that the API has returned via Order_GetLines looks like this:

object->Id = 292
object->Number = 2

I think the last value should be 1 and not 2. The problem occurs in some cases where I have removed orderlines from the order using the API... but not in all cases.

Update: OrderLine_GetData also fails.

Most likely the real problem lies in Order_GetLines, because i returns a wrong number (2) for the one and only orderline on the order.

Ok, I figured it out: OrderLine_GetDataArray only returns an array if there are multiple lines. Same goes for Order_GetLines.