Hi,
Glad you found it, that is exactly the cause.
The "Override registration" option in Configuration > Checkout is meant for merchants who want to replace HikaShop's registration and guest forms with their own content. When it is set to Yes, HikaShop stops outputting the whole registration/guest form block (the account fields, the email field and the address fields all live there) and instead displays the text entered in the field right below that option. When that text is left empty, the area simply renders blank, which is why the fields looked missing and buyers could not get past the login step.
Setting it back to No restores the standard register and guest forms with the email and address fields, and the checkout works normally again.
It is not toggled by the update itself, so it had been turned on at some point in your configuration and the effect only became visible to you now.
