Hi,
Well, there is no reason that K2 would display that error when you try to register with HikaShop. I think they should either add a whitelist of places where it's possible to display the K2 captcha and process its answer, or a blacklist of such places where they shouldn't process anything (and put HikaShop in it), or support the HikaShop registration so that their captcha is displayed on the HikaShop registration form (and thus you wouldn't use HikaShop's catpcha in that case).
But I don't see what we can do in HikaShop to prevent that. I guess you'll have to check with them what they propose.