Sending e-mail with new comment failed - INVALID_ID_FIELD
Error: Sending e-mail with new comment failed: SendEmail failed. First exception on row 0; first error: INVALID_ID_FIELD, WhatId is not available for sending emails to UserIds.: 
There is a Salesforce platform requirement that to send an email with a Text or custom HTML type template, you must specify a contact for it to be related to. You can get around this by using one of the Visualforce type templates that we include. The caveat with that option is that it will include the Current User as a recipient if the Case Contact is not selected.
Text and custom HTML templates require a Contact while Visualforce templates require either a Contact or User.
Email to Case Premium comes packaged with a configuration option called "Contact Substitution" to work around this requirement. Step 2 of the (initial) Setup Wizard will take you through configuring Contact Substitution.
The issue is resolved by setting up Contact Substitution