Comment cannot be empty
The comment API rejected a post/edit because there was no usable text after trim (and after banned-word masking).
Why this happens
Whitespace-only input, or Bad comment words replaced so much of the body that nothing remains. Applies to file download comments, public folder comments, and news comments that share the same validation.
Diagnose and fix
1. Visitor: type a real comment and submit again.
2. If legit text vanishes: Settings → Files → Bad comment words filter (bad_comment_words) — trim over-broad terms.
3. Confirm comments are enabled for that surface (otherwise you see Comments are disabled first).
4. Not a captcha error — empty body is checked on its own.