I originally had Tweet Old Posts installed. I had an issue with it and thought I would try Tweetily. This had the same problem so I deactivated the plugin and deleted the files. I then installed Tweet Old Posts but when I start that plugin it is populated with all the settings from Tweetily still, even though I deleted that plugin.
It puts the http://mydomain.com/wp-admin/admin.php?page=Tweetily address into the Tweet Old Posts settings page and if I change it and try and save the settings it says I can't authorize it!