Hi,
That's because the selection of weither to login the user or register it or guest register it depends on the button clicked (login, or register, or next) and if the main button of the page is clicked, then the system can't relies on that. In that case, it relies on whether the login fields are filled in or the registration/guest form fields are filled. And when nothing is filled, the system defaults to login.
We'll add some additional check to properly handle that in the next version of HikaShop.
In the mean time, in your case, you can simply use a translation override in order to check the error message there.