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If you are dealing with the issue of Yahoo mail on Android device and want to know about solutions to fix cannot sign into Yahoo mail on Android device then it might be because app function slow or crashing. Start by ensuring that operating system of Android device is running with the latest version. Ensure to clear app data and cache Yahoo mail app. To do it, proceed to settings > apps > manage apps > Yahoo mail > clear data. After that, you need to force stop and restart the application. For this, go to settings > apps > manage apps > Yahoo mail > force stop. After force stopping the application, restart it and check whether the issue is cleared or not.