Activity NoClassDefFoundError using Android Studio

Note: This is not a missing class in a jar, so don't rush to mark this as a duplicate.
I'm porting a big project from eclipse to Android Studio. I finally succeeded to see my SplashActivity on screen, but I fail to get the MainActivity which is launched after my SplashActivity. I'm getting the following error on runtime:

> 06-13 14:42:30.176 12389-12389/com.totalboox E/AndroidRuntime: FATAL
> EXCEPTION: main Process: com.totalboox, PID: 12389 
>    java.lang.NoClassDefFoundError: com.androidcore.android.main.MainActivity$4
>    at com.androidcore.android.main.MainActivity.<init>(MainActivity.java:587)
>    at java.lang.Class.newInstance(Native Method)
>    at android.app.Instrumentation.newActivity(Instrumentation.java:1067)
>    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2317)
>    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
>    at android.app.ActivityThread.-wrap11(ActivityThread.java)
>    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
>    at android.os.Handler.dispatchMessage(Handler.java:102)
>    at android.os.Looper.loop(Looper.java:148)
>    at android.app.ActivityThread.main(ActivityThread.java:5417)
>    at java.lang.reflect.Method.invoke(Native Method)
>    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
>    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

MainActivity is declared in the Manifest:

<activity
    android:name="com.androidcore.android.main.MainActivity"
    android:label="@string/app_name"
    android:theme="@android:style/Theme.Holo.Light"
    android:windowSoftInputMode="stateHidden|adjustPan"
    android:launchMode="singleTop">
    <intent-filter>
        <action android:name="com.totalboox.MESSAGE"/>
        <category android:name="android.intent.category.DEFAULT"/>
    </intent-filter>
</activity>

The code at MainActivity:587

// Pushwoosh lib integration (see https://www.pushwoosh.com/programming-push-notification/android/native-android-sdk-integration/)

BroadcastReceiver mBroadcastReceiver = new RegisterBroadcastReceiver()
{
    @Override
    public void onRegisterActionReceive(Context context, Intent intent)
    {
        checkMessage(intent);
    }
};  
Where RegisterBroadcastReceiver is from an external lib (pushwoosh) handling notifications.

The code in SplashActivity starting MainActivity:

Intent mainActivity = new Intent(this, MainActivity.class);
mainActivity.putExtra("deep_link_book_id", deepLinkBookId);
startActivity(mainActivity); 
finish();

Googling this I found people having this problem with a jar. In my case MainActivity.java is one of my source modules, and its package name is package com.androidcore.android.main; exactly as declared in the Manifest. The project is built of 4 subprojects. SplashActivity & MainActivity are in the same subproject.

I've spent countless hours on this. I tried: Clean, Rebuild, Invalidate Cache and Restart - nothing helped. Anyone can advise how to stop this nightmare? Many thanks in advance !