After clicking Send, the button grays out. No emails are sent and the page does not refresh.
The underlying issue is a jquery error: Uncaught TypeError: Illegal invocation. This is due to incorrect formatting in the HTML for an approved email. The insertEmailFragments token is not in its own table.
Place insertEmailFragments in its own table per the instructions in the Product Help documentation.
CRM Help Documentation: Approved Email Configuration Tokens