It was a bug when during editing language key and or email template all tags was stripped. 

So I would suggest to try to restore it to the default by editing '_sys_et_txt_body_confirmation' language key in Studio > Polyglot > Keys.

Enter the following code for the language key value:

{email_header}
<p>Hello, {name}.</p>
<p>Please verify your email address by clicking on the link below:</p>
<p><a href="{conf_link}">Verify email address</a></p>
{email_footer}