Request permissions not working within PermissionsPlugin

For my Xamarin.Forms-App I'm using the PermissionsPlugin v2.0.1.

If no permission is granted, I want to request the user to do so by CrossPermissions.Current.RequestPermissionsAsync(Permission.Storage)

The problem is that the function always returns Unknown instead of requesting the user for the permission. One time, it worked...but I don't know what I changed that it's not working anymore..

Here my code:

MainActivity.cs (Android-project)

public override void OnRequestPermissionsResult(int requestCode, string[] permissions, Permission[] grantResults)
{
    base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    PermissionsImplementation.Current.OnRequestPermissionsResult(requestCode, permissions, grantResults);
}

AndroidManifest.xml

<uses-sdk android:minSdkVersion="15" android:targetSdkVersion="25" />
<uses-permission android:name="android.permission.CAMERA" />

PermissionsExample.xaml.cs

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class PermissionExample
{
    public PermissionExample()
    {
        this.InitializeComponent();
    }

    private async void Button_OnClicked(object sender, EventArgs e)
    {
        var requestedPermissions = await CrossPermissions.Current.RequestPermissionsAsync(Permission.Camera);
        var requestedPermissionStatus = requestedPermissions[Permission.Camera];

        await this.DisplayAlert("", $"Permission: {requestedPermissionStatus}", "ok");
    }
}

Thanks for your help!