Problem copying my stuff from hololens to OneDrive StorageFolder.GetFolderFromPathAsync returns null

Hello,

I want to export multiple files from my HoloLens own unity app, stored here:

Globals.DataPath = Windows.Storage.ApplicationData.Current.LocalCacheFolder.Path;

to OneDrive. So I try to open a FolderPicker to get / choose the OneDrive Folder for copying my stuff into, but it always return null: See current Code, saveFolder always null. I also tried to wait and reinstalled OneDrive on HoloLens, nothing helps.

Code (CSharp):

1.     private async void CopyFolderAsync()
2.     {
3.         UnityEngine.WSA.Application.InvokeOnUIThread(async () =>
4.         {
5.             FolderPicker saveFolderPicker = new FolderPicker();
6.             saveFolderPicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;
7.             saveFolderPicker.FileTypeFilter.Add("*");
8.             Globals.GuiManagerInstance.SetPopupText("Export gestartet,\nbitte warten.");
9.             StorageFolder saveFolder = await saveFolderPicker.PickSingleFolderAsync();
10.             if (saveFolder != null)
11.             {
12.                 StorageFolder sourceFolder = await StorageFolder.GetFolderFromPathAsync(Globals.DataPath);
13.                 if (sourceFolder != null)
14.                 {
15.                     IReadOnlyList<StorageFile> filesInFolder = await sourceFolder.GetFilesAsync();
16.                     foreach (StorageFile item in filesInFolder)
17.                     {
18.                         Debug.LogWarning("file found: " + item.DisplayName);
19.                         StorageFile copyfile = await item.CopyAsync(saveFolder);
20.                         //await item.CopyAndReplaceAsync(item);
21.                     }
22.                     if (filesInFolder.Count == 0)
23.                     {
24.                         Debug.LogError("Export: No files in sourceFolder");
25.                     }
26.                 }
27.                 else
28.                 {
29.                     Debug.LogError("Export: Error getting sourceFolder");
30.                     Globals.GuiManagerInstance.SetPopupText("Export fehlgeschlagen,\nbitte noch einmal probieren.");
31.                 }
32.             }
33.             else
34.             {
35.                 Debug.LogError("Export: Error getting saveFolder");
36.                 Globals.GuiManagerInstance.SetPopupText("Export fehlgeschlagen,\nbitte noch einmal probieren.");
37.             }
38.         }, false);      
39.     }

I gave my app on the HoloLens every single permission, in unity Player Settings, I use Unity 2017.3.1f1 and Visual Studio 2017 for deploy. Scripting backend is .Net, I tried various Api Level.

Any idea what am I doing wrong?

The FileSavePicker or FileOpenPicker seem to work and open the OneDrive app, but the FolderPicker never opens anything and always Returns null...

Please help

Thanks!