When syncing an invoice or bill to QB you may get an error message indicating that a customer, item or vendor was not found in the list OR invalid argument.
1) As the AO administrator, go to Company -> Synchronize QuickBooks -> Sync Queue Manager
If this record shows on the SyncQ please delete it from the Queue. This will not delete the record from within its list of similar entities..i.e., it will remain within the customer, item, vendor list..
If this record does not show on the SyncQ, proceed below.
2) Go into QB and create the customer, item or vendor (said not to be on the list) with all the same attributes that were used to create it in AO, such as the gl account settings, etc.
3) Do another synchronization
4) Now reopen the customer, item or vendor, edit it if needed and save
If the steps above do not work, proceed below.
Login to QuickBooks as the Admin user in single user mode. For an Item, Go to Lists -> Item List -> Towards the bottom click on Item -> Re-sort List and click OK. For a Customer, Click on Customer Center -> Right-click on the customer list -> Re-sort List and click OK. Then try syncing again.