Symptoms

You received the following error in your comment notifications:

The error is only observed when someone commented on a page and you received the notification. It doesn't seem to affect any other mail notifications.

Diagnosis

Temporarily enable Plugin Safe Mode and see if it makes any difference. This will disabled all user-installed plugins at once. So if the issue does not occur again, it is caused by one of the third-party plugins installed. This may need to be done during non-peak hours as disabling third-party plugins would cause users unable to perform some operations provided by those plugins. Once the test has been carried out, disable this mode.

Cause

Caused by Confluence InPlace Editor

Resolution

Disable the plugin and contact the author directly at http://stiltsoft.desk.com/customer/portal/emails/new