Woocommerce Checkout error caused by Mailjet

Hi there,
We experienced a wc-ajax 504 error on woocommerce ckeckout page. When the user finally clicks to place the order the page keeps loading and a general error comes out. In the console we see a 504 error for the wc-ajax=checkout call. The payment succeed but no confirmation is shown to the user (while he recceive a “Error Processing Checkout. Please try again.” error)

Deactivating mailjet plugin solved the problem.
That happens with default themes (storefront).

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