Manifest Merger not working

Question

I think the manifest merger doesnt work for me. Im running latest Android Studio 2.2.3

My Project has :App - Application :Library - Library Project

The Library has all activities. The app shall just use it and launch its default activity. Im just getting this error msg:

Error running App Default activity not found

This is my apps manifest:

<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="trainer.de.next"
    xmlns:tools="http://schemas.android.com/tools">

</manifest>

The merged manifest shows not more, this is the report:

Manifest Sources 
app main manifest (this file)
 Other Manifest Files (Included in merge, but did not contribute any elements) multidex:1.0.1 manifest

In the build.gradle of app im using the library

...
dependencies {
    compile project(':library')
}