Error while sending XML to UPS. Error code: 250002

-- url of the page with the problem -- : test.atrnameh.com
-- HikaShop version -- : 2.4.0
-- Joomla version -- : 3.4.1
-- Error-message(debug-mod must be tuned on) -- : Error while sending XML to UPS. Error code: 250002

Hello,

I tried to configure UPS plugin with the credential information my customer provided. But I faced this error in checkout page:
Error while sending XML to UPS. Error code: 250002. Message: Invalid Authentication Information

I sent my problem to UPS support as I thought it is probably not a HikaShop issue but authentication problem with my credentials and they asked me for "XML request and response files associated with the error".

I checked UPS plugin files and could not find such XML files. It would be very appreciated if you could help me about how to obtain these files and any other suggestion about how to deal with this error would be very appreciated.

Thank you.

Dear Jerome,

Thanks for your response. Unfortunately I could not find the "debug" option in UPS plugin settings. But I could obtain the required XML info by adding two echo commands in lines 1726 (XML request) and 1880 (XML response) of this file:
plugins > hikashopshipping > ups > ups.php
Command: echo $xml;

I will explain the complete solution in details as soon as I can solve it.