I set Mahara to integrate with Moodle, so that with a single (or more?) click, one can export their work to their Mahara portfolio.
Because of the strange way we do terms in Moodle (each new term is a new Moodle instance with a different URL), the public key was invalid (it’s encoded with the base URL) each time we changed terms.
To fix this, simply go into Moodle (Networking / Settings) and delete and regenerate the public key. This will create a new key encoded with the new, base URL. Copy this key and login to Mahara as admin, and visit Administration / Institutions and edit the settings for your institution. Specifically, change the URL and update the public key under the XML-RPC authentication settings.