Error in Inventory Quantity Field in Accountright Plus

We have started to have problems with some inventory items that have been bought and sold previously, and currently with a nil inventory balance. Items in Inventory have the correct 0 [zero] balance but when the item is entered on a new purchase order the inventory amount comes up with a strange amount eg 89209850063E-16 [please attahced screen grab] similar to an Microsoft Office Excel field. When you come to invoice the item in purchases MYOB freezes and requests to close down, losing connection to our network and forcing a restart.

Please assist as this is time consuming and frustrating.