We did make some changes back in June. Here is one of our announcement posts on the subject:
If you missed this announcement, I suggest that you ‘watch’ our announcements category by clicking the small button in the top right hand corner of the category, and to check out our stay informed page on our documentation.
You are going to have to go into the apps dashboard via apps.twitter.com (or if you have applied for a developer account, via the new dashboard at developer.twitter.com), and add your callback URLs to the list.