I upgraded my Android studio to 1.4. Now when I'm deploying my app to my device I get the message that the device has an application with the same package but a different signature. How is this possible? I already did some research and I looked up my debug.keystore, but this hasn't changed since 13/03/2015. Anyone has any idea how I can fix this?
Debug builds are signed with the SDK keystore which is generated when you install the Android SDK. On Linux / OSX it is located at ~/.android/debug.keystore.
The message you had means that you have a debug build installed and you want to install an App Store build on it (or the other way around).
This can also happen when you deploy from a different machine.