If you have not modified the templates, then the closing comment tag should be there.
If it’s not found, this is likely due to an error being interjected with the HTML. This is especially likely if you have not set
WP_DEBUG_DISPLAY to false in your wp-config.php. Did you check the debug.log for errors?
In any case, please do the following:
1. Ensure that you are not modified the templates.
2. Let me know which shortcode you are using, and with what attributes exactly.
3. Email me a regular user username and password, and the link to the page, so I can login and check the issue on your site.