Android - [...] emulator-5554 disconnected! Cancelling '... activity launch'!

 Every time I run an Android application, I get this error "[...] emulator-5554 disconnected! Cancelling '... activity launch'!". Doesn't occurs when I ran an app for the first time.

Android Debug Bridge (adb) is a client-server program with (adb) client and (adb) server components running on the development machine and a daemon process running on the emulator or device. Chances are that if the communication between any of the components fails or takes longer, a timeout migth be flagged. Causing a cancellation of app activity.

Solution (Eclipse IDE)
  1. Select & Right Click on Android Project
  2. Run Configurations
  3. Go to tab Target 
  4. Enable option Wipe User Data on Emulator launch parameters
  5. Run Application

If you are using other IDE, you can restart your emulator using -wipe-data flag to delete all the temporary files that the emulator created in previous runs.