Hi sorry to re-up an old post put I got the same problem.
Here is the solution :
In the plugin file \plugins\authentication\email\email.php
The code to get the username is :
$username = JFactory::getApplication()->input->post->get('username',null,'RAW');
But during the submit of the login form in hikashop the login is not provided by a field name "username".
To adjust the code you have to change this line by :
$username = JFactory::getApplication()->input->post->get('username', isset($credentials['username']) ? $credentials['username'] : null ,'RAW');
This should fix the problem in all situations.
Best regards.