Hi Lars,
Check the version of Adagio Order Entry in use - we made a fix in this area for the April 2006 SP. The issue had to do with posting an order with qty shipped > 0, then coming back at a future date to invoice it. The problem occured if the order was simply edited and click OK to post. The problem didn't happen if the 'Ship All' ('Invoice') button was clicked first, or if any items were edited.
It may be that they don't have the most current service pack installed. Or when they installed it, their shortcut for starting Adagio OE got redirected by Windows to the \softrak\oewin\backup directory. Thus they aren't actually running the most recent version.
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Regards,
Softrak Tech Support