I haven't been working in Android Studio last 2 weeks, now i come back to change something in my app, and i can't debug apps anymore on my smartphone.
I get this Event Log:
21:55:16 Gradle build finished in 4 sec
21:55:16 Session 'app': running
21:55:20 Session 'app': error
Finally solved it.
I was experimenting something with my default install location before. At some point i set instal-location to 2, which means external storage (SD card). This totally screws AS, so i had to revert it:
Run cmd from your sdk folder:
adb devices adb shell pm set-install-location 1
Edit: As you can see from other answers you can get same error for other issues. You might want to look other answers and find what could be your problem.