Seeing this ^ error and currently don't know how to fix it. It happens only on some of my (presumably identically configured) websites. This is happening when a user attempts to post any comment.

Turns out, this was due to a workflow rule that attempted to email the author of a parent-comment and notify them that their comment has been replied to. Since there is no parent comment, there is no email to send a message to, which was causing the error.

I temporarily disabled the workflow rule, while I figure out how to email the owner of a comment *only if* the comment is a reply to another comment.