Comment-Emailer allows you to customize the email wordpress automatically sends when a new comment, pingback or trackback has been approved.
No, this version is using the default wordpress function which only sends text emails.
Sorry, this issue has been fixed but if you have an older version please disable the plugin before upgrading. If you have already tried to upgrade and get this error you will need to edit one of the files.
Go to plugin - editor and from the upper dropdown select comment-emailer then edit the top file comment-emailer/comment-emailer.php
Line 22 says include 'notify_postauthor.php'; please comment this line out activate the plugin then disable the plugin. Once you have diasabled the plugin you can uncomment this line and then reenable the plugin.