Internal Server Error
You guys probably already know that there is a problem with your latest release of Email Customizer, but I'm going to submit this anyway.
The Email Customizer plugin was creating an "Internal Server Error" on every order that was being placed on our website. I tested it several times, and received the same "Internal Server Error' (see screen shot). http://www.institutebooks.com/wp-content/uploads/2016/04/Screen-Shot-2016-04-01-at-2.46.57-PM.png
There is no preview of the email template when I try to modify it, and no emails are being sent. Also, the preview of every email in the WooCommerce order screen does not work.
Since the plugin wasn't showing the preview(s), and wasn't allowing me to modify the template(s), I figured it is what was causing the Internal Server Error message on every order. So, I deactivated your plugin, and our website is able to take orders just fine. Please fix!