Google Maps Activity V2 Android Studio

I have troubles with integrating Google Maps API v2 into my app.

I'm using Android Studio. I created a new maps activity and followed the instructions stated in the google_maps_api.xml:

<resources>
    <string name="google_maps_key_instructions">
    <!--
    TODO: Before you run your application, you need a Google Maps API key.

    To get one, follow this link, follow the directions and press "Create" at the end:

https://console.developers.google.com/flows/enableapi?apiid=maps_android_backend&keyType=CLIENT_SIDE_ANDROID&r=XXXXhashXXX%3Bat.schickermueller.taxameter.activity

    You can also add your credentials to an existing key, using this line:    
    XXXXhashXXX;at.schickermueller.taxameter.activity

    Once you have your key (it starts with "AIza"), replace the "google_maps_key"
    string in this file.
    -->
    </string>
    <string name="google_maps_key" templateMergeStrategy="preserve">AIzaXXXXXXXXXXXXXXXXXXXXX</string>
</resources>

I also activated the Android maps v2 API:

but the activity shows a blank map and there is the following error:

08-23 12:30:45.845 11659-11829/at.schickermueller.taxameter E/Google Maps Android API﹕ Failed to load map. Error contacting Google servers. This is probably an authentication issue (but could be due to network errors).

I already tried some 'solutions' posted in stack overflow threads(like Android - Google maps Api v2 - Authentication error)

My manifest has the following content:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="at.schickermueller.taxameter" >

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".activity.StartActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <service
            android:name=".service.TrackService"
            android:enabled="true"
            android:exported="true" >
        </service>

        <activity
            android:name=".activity.DriveListActivity"
            android:label="@string/title_activity_drive_list" >
        </activity>
        <activity
            android:name=".activity.DriveDetailActivity"
            android:label="@string/title_activity_drive_detail"
            android:theme="@style/Theme.AppCompat" >
        </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" />

        <activity
            android:name=".activity.MapsActivity2"
            android:label="@string/title_activity_maps_activity2" >
        </activity>
    </application>

    <permission
        android:name="at.schickermueller.taxameter.permission.MAPS_RECEIVE"
        android:protectionLevel="signature" />

    <uses-feature
        android:glEsVersion="0x00020000"
        android:required="true" />

    <uses-permission android:name="at.schickermueller.taxameter.activity.permission.MAPS_RECEIVE" />
    <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_FINE_LOCATION" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.READ_PHONE_STATE" />
    <!--
         The ACCESS_COARSE/FINE_LOCATION permissions are not required to use
         Google Maps Android API v2, but are recommended.
    -->
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

</manifest>

I hope you can help me