Hi,
I think multiple payment method would not be the problem.
Check.
a. KB 3210698 and KB 3187456
b. Verify/Debug: RetailTransactionService::findDuplicateSalesOrder .Perhaps, by reverse engineering, you can identify some non-compliance in the code vs. the data in your sales order. (salesTable/retailSalesTable)
c. Option to test: in pos.exe.config increase the value tag sendTimeOut
I Hope this Helps!
Regards,
Rodolfo Recalde