Sometimes you may just see "Loading...." message, especially after saving your email template.
This happens when you are having any security plugins that dis-allows saving html.
Please check if you are using any security plugins like SecurePress or WordFence, that disallows saving html.
The email template is saved as a JSON entity including the HTML design.
Since the email template is a html design, the html tags are present in the JSON string. Sometimes your security plugin would not allow it.
Here is an easy way around it
Step 1: Reset that particular template (which shows Loading message)
Please go the Email Customizer plugin settings -> Reset templates.
Choose the template in the dropdown and click re-set.
Step 2: Check the wp-config.php for the following constant:
Most security plugins might have added the following constant to your wp-config.php file.
Edit wp-config.php file
Find this line
define( ‘DISALLOW_UNFILTERED_HTML’, true );
Comment it out
// define( ‘DISALLOW_UNFILTERED_HTML’, true );
Once you have completed all your design work in the email template and saved, you can uncomment this line.