Error when trying to 'receive' a PO

Hello All,

We are experiencing an error in AX 2012 R3 "Procurement and Sourcing" module.

When we try to click on 'Product receipt' under "Receive" tab, we get the following stack trace. Any ideas as to how to resolve this?

thank you

(S)\Classes\PurchFormLetterPackingSlipContract\unpack - line 9
(S)\Classes\SysOperationDataContractInfo\unpackDataContractObject - line 21
(S)\Classes\SysOperationController\unpackDataContractInfoObjects - line 34
(S)\Classes\SysOperationController\unpack - line 82
(S)\Classes\SysOperationServiceController\unpack - line 15
(S)\Classes\PurchFormLetter\unpack - line 10
(S)\Classes\PurchFormLetter_PackingSlip\unpack - line 148
(S)\Classes\xSysLastValue\getLast - line 29
(S)\Classes\SysOperationController\loadFromSysLastValue - line 29
(S)\Classes\SysOperationController\getDataContractInfoObjects - line 10
(S)\Classes\SysOperationController\getDataContractInfoObject - line 17
(S)\Classes\SysOperationController\getDataContractObject - line 18
(S)\Classes\FormLetterServiceController\getLast - line 3
(S)\Classes\FormLetterServiceController\getLastFromCallerFormName - line 18
(S)\Classes\PurchFormLetter\mainOnServer - line 49
(C)\Classes\PurchFormLetter\main - line 50
(C)\Classes\FormFunctionButtonControl\Clicked