You are using Email to Case Premium and experience the following error when sending a new comment:
pg:addCommentF:addCommentPB:rptOrder:4:templatePBS:templatePBSI:template_Select: Validation Error: Value is required.

Upon sending a case comment, the text you've typed in the comment area will be merged into an email template and subsequently sent to the recipients. This error (or the inability to change the Email Template in the Template section) indicates that the running user does not have sufficient access to the email template folder defined in the Email to Case Premium settings.

To confirm email template folder you've defined in the Email to Case Premium settings, select Email to Case Premium from the app menu and click the tab titled Email to Case Premium Settings. From there, scroll down to the section titled Email Templates and confirm the folder selected.

Upon obtaining the name of the folder, navigate to Setup | Administer | Communication Templates | Email Templates. Select the matching folder from the picklist and click the Edit button.

From here, you can adjust the folder access to provide sufficient access to the affected users.