Have been working with several NAV-versions - never experiencing that problem. And 3.70 was (and still is) a “good”, the 3.70b at least.
But I have neve worked that much with Warehouse - only quite plain Purchase and Sale; if you are using Warehouse (picks and put-aways) - could it be something there?
Do you use Reservation or Drop Shipment/Special Order? Using those two cause posting of a [purch] shipment when posting the connected sales order. I have used theese two functions, but without any problems.
this is not a standard Navision error message. Is this the exact message, or did you translate it? If this is the exact message, then my guess is that this is a customization and that could indicate the reason for the error. If this is the case you need to talk to your partner to have them rectify it.