I have finally bitten the bullet and upgraded the web forum to the latest version of phpBB, dropping the m2f module in the process. The m2f module makes pretty deep modifications to phpBB's core, which makes it more difficult to update phpBB regularly. By dropping m2f, I hope I can keep phpBB more up-to-date.
What does all this mean to you? It means you will no longer be able to subscribe or interact with the web forum via email. Very few people do that any way, so if you are currently not using the web forum's email function, you can ignore this message. However, if you are currently subscribed to the web forum via email, I would suggest that you switch to tracking the web forum via RSS feeds.
Finally, to all those affected, I would like to sincerely apologise for any inconvenience caused. I have been considering and reconsidering this decision for a very long time, and I hope it's all for the best.