Access Denied / Page Not Found when posting comment in alternative language, works in primary language

This is tested on 6.x-4.0-beta7 and 2.3.

Posting a comment to a content type where notifications, permissions, etc. are set up correctly, everything works fine.

When viewing the very same node in a secondary language, say, French, the URL looks like this:
www.domain.com/fr/node-name

When a comment is posted, the URL becomes www.domain.com/fr/fr/node-name, which results in an "Access Denied" error, where the referrer is the original working URL.