You can’t get a conclusion like that by limited information. Another thing is that data and settings are different on those two sites, which causes the result you got was wrong.
PrestaShop 1.7.2 is a unstable version, its’ has a problem of not checking stock, which means it’s possible to order out of stock products, that’s what happens on your site. You can google the bug, you can also ask your technical guys to compare this file with the one comes from 184.108.40.206 to see the bug https://github.com/PrestaShop/PrestaShop/blob/develop/classes/checkout/CheckoutPaymentStep.php
Because the bug, the native checkout process can work, but the easycheckout module can’t.
I’ve removed the feature to check stock information for your easycheckout module to make it be able to work.
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