Re: [Mailman-Developers] Update MM 2.1.2->2.1.5 fails and no help from mailman-users

If I recall correctly, Mailman still doesn't always deal gracefully with old pending stuff, so you may have to clear out those files before doing the upgrade (as well as upgrade python, as was already mentioned).

This used to be mentioned in the README or INSTALL files, and I'd assume it's still buried in there somewhere, although it's hard to blame you for missing it.