This is usually caused by having too many numbers in the QTY field of an invoice (the QTY field says 2050000 for example).
KC should tell you the invoice getting the error. Go to Accounting, Invoices, open the Invoice and make sure the QTY field has the correct number. If it doesn't, change it to the right quantity and sync again.
If that does not fix or if you cannot find the right invoice, contact Support for assistance. Support@SoftwareConnectionLLC.com or 1-888-486-4343 option 2.