Hi Mark!
Yup, I'm using the Unsubscribe page that was designed for the system. It looks like it used to work and is just an issue that appeared in October. Here's what one of the recent unsubscribed lead's Activities looks like:
Seems like the fail is happening when Marketo tries to sync the Interesting Moment to SFDC. We don't have MSI installed yet, so I'm not sure what else could have happened to break the sync recently.