Nabble2 shows 404 when subscribing to a forum

How to reproduce:

- Go to http://n2.nabble.com/Plone---Poland-f1461.html

- Click "New Topic" (leads to http://n2.nabble.com/forum/NewTopic.jtp?forum=1461)

- Get the "The mailing list may require your subscription before accepting your post" reminder, click "Subscribe"

- Be greeted with the less-than-friendly error message:

  HTTP ERROR: 404

  NOT_FOUND
  RequestURI=/forum/Subscribe.jtp

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