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I found a bug in multilanguage sites with url prefix activated.
uc_referer_check in uc_store.module is not able to validate the referer correctly as it's not taking into account the language url prefix, therefore not validating the order review and sending an empty cart error to the user.
The user can not complete the checkout if they are in an alternate site language.
I made a quick fix but it's not a very elegant solution.