Several third-party Twitter clients are experiencing technical difficulties, preventing users from logging in. Tweetbot and Twitterific have both confirmed the issues and are working to find the root cause. Fenix, an Android app, has also confirmed that it is experiencing problems, but its iOS app seems to be unaffected. Matteo Villa, the developer of Fenix, has said that the Android app was suspended with no communication from Twitter.
Villa posted a screenshot showing a notification that Fenix violated Twitter Rules and policies and can no longer be accessed. The developer has temporarily pulled the app from Google Play while trying to determine the cause.