Twitter Tools user here. Upgraded this AM and installed Social to facilitate. Can't connect the Twitter account.
Turned on debug and added some of my own debug, and it appears as though Sopresto.mailchimp.com isn't POSTing back the data needed to establish the OAuth connection. The POST data is completely empty, as opposed to having the id (nonce), keys user, or service values.
Upon going to my Twitter account's Apps page, it does show the app "Social Proxy by Mailchimp" as authorized.