Android : Android Google Map v2 signed apk doesn't work

Yet another question on GoogleMaps Api Key and release mode.

I have my API key, correctly added the SHA-1 in the allowed Android Applications, copied the key in the manifest and the same in both google_maps_api.xml (debug, release) in Android Studio. In debug everything works fine, but when I install the app through the Signed Apk it shows me gray screen and google logo. Anyone knows what I can do about that? Any help would be much appreciated. This is my AndroidManifest:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />

<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.VIBRATE" />
<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
<uses-permission android:name="android.permission.GET_ACCOUNTS" />
<uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />

<permission
    android:name="com.permission.C2D_MESSAGE"
    android:protectionLevel="signature" />

<uses-permission android:name="com.permission.C2D_MESSAGE" />

<application
    android:name=".app.AppController"
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".Welcome"
        android:configChanges="orientation|keyboardHidden|screenSize"
        android:label="@string/app_name"
        android:screenOrientation="portrait"
        android:theme="@style/FullscreenTheme" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name=".MainActivity"
        android:label="@string/title_activity_main"
        android:screenOrientation="portrait" >
    </activity>
    <activity
        android:name=".MenuActivity"
        android:label="@string/title_activity_menu"
        android:screenOrientation="portrait" >
    </activity>
    <activity
        android:name=".GalleryActivity"
        android:label="@string/title_activity_gallery"
        android:screenOrientation="portrait" >
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.MainActivity" />
    </activity>
    <activity
        android:name=".TripActivity"
        android:label="@string/title_activity_trip"
        android:screenOrientation="portrait" >
    </activity>
    <activity
        android:name=".PromoActivity"
        android:label="@string/title_activity_promo"
        android:screenOrientation="portrait" >
    </activity>
    <activity
        android:name=".ContactActivity"
        android:label="@string/title_activity_contact"
        android:screenOrientation="portrait" >
    </activity>

    <meta-data
        android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />
    <meta-data
        android:name="com.google.android.maps.v2.API_KEY"
        android:value="@string/google_maps_key" />