This error may occur when trying to update information when either saving an entity or syncing with Quickbooks. Typically this is caused by an option being selected from a drop down and assigned to an entity and the item selected from the drop down does not exist in Quickbooks.
If trying to update a customer or vendor check that any assigned customer/vendor type, ship method, payment method, tax codes, terms, sales reps, currency etc... exist in Quickbooks in their proper lists.
If trying to update an item check that any assigned classes, tax codes, accounts, default vendors etc... exist in Quickbooks in their proper lists.
If you find a missing entity manually create it in Quickbooks and go to the Company menu, Synchronize Quickbooks, Selective Sync and check the box for the entity type and run the Sync. Then try and update the original entity again. If you cannot find it run the selective sync and check all boxes and then try and update the original entity again. If the original entity is trying to update via the sync queue be sure to remove it from there before doing your selective sync.