mailman archive failure due to bad group permissions

I’ve had a persistent inability to access archived mails using mailman 2.1, the stock Virtualmin install, currently on Ubuntu 14.04.3
A lot of casting around in forums and search engines has resulted in me seeing, here: https://answers.launchpad.net/ubuntu/+source/mailman/+question/249403 Mark Sapiro’s agreement that a useful diagnostic is to run mailman’s bin/check_perms. When i do I get 200-odd messages of this general form:

/var/lib/mailman/archives/private/stewards/attachments bad group (has: www-data, expected list)

I can and will check and change permissions and ownerships as indicated in the diagnostic output.

I don’t understand how this arises. The affected lists are of various ages and some have been migrated from earlier versions of Virtualmin and from an older CentOS install. In particular, if there’s something I can do in future to avoid this situation I’d like to know about it.

Anyone have advice? Is it a bug?

Dave