For a second weekend in a row, Twitter has put up defenses against a denial-of-service attack that also block all legitimate high-volume API calls from third-party applications such as TweetLater.
This issue started on Saturday afternoon (August 15th), and has been ongoing since then with very little support from Twitter and no communication about it on any of their status sites.
Please monitor the service notice that is posted above the blue menu tabs on TweetLater.com, which you will see after you have logged in to your account.
We will also publish updates here until the issue is resolved.
Update, Monday, August 17th, 1:39 PM EST: At 1:39 PM EST (10:39 AM PST) we started seeing activity from the Twitter engineers, saying that they were beginning to look into the issue.
Update, Monday, August 17th, 5:42 PM EST: Unfortunately, there is still no indication from Twitter regarding a timeframe for them solving the issue. Several other applications are affected by the same issue and other API issues at present. Some are rendered completely inoperative, while others can only limp along at best.
Update, Monday, August 17th, 8:15 PM EST: We have now enabled all Twitter automation. The Twitter API appears to be functional again.