Upload a large file into Shared Folder through Microsoft Graph API app only asp.net Access Denied - asp.net

Question

I'm following the example of the aspnet-snippets-sample
and OneDriveTest.cs to upload a large file through the Microsoft Graph API into a shared folder with app only credentials
and following exception is thrown:
"Access denied. You do not have permission to perform this action or access this resource."
To initialize a large file upload session I do following:
var uploadSession = await _graph
.Drives[DRIVEID]
.Items[ITEMID]
.ItemWithPath("newFile.docx")
.CreateUploadSession()
.Request().PostAsync();
Both the driveId and itemId are valid.
I'm creating my GraphClient like following:
public MicrosoftGraphHelper()
{
_graph = new GraphServiceClient("https://graph.microsoft.com/beta",
new DelegateAuthenticationProvider((requestMessage) =>
{
requestMessage.Headers.Authorization = new AuthenticationHeaderValue("bearer", AccessToken);
return Task.FromResult(0);
}));
}
The access token is acquired through my app credentials and secret. In Azure, my app has read/write permissions for files through Microsoft Graph.
Also, creating a new folder like
POST https://graph.microsoft.com/beta/drives/DRIVEID/items/ITEMID/children
with JSON
{
"name": "NewFolderTest",
"folder": { }
}
works as well.
here is the Upload Task
internal async Task<string> UploadLargeFile(HttpPostedFileBase file)
{
try
{
using (Stream fileStream = file.InputStream)
{
// Create the upload session. The access token is no longer required as you have session established for the upload.
// POST /v1.0/drive/root:/UploadLargeFile.bmp:/microsoft.graph.createUploadSession
var driveItems = await _graph.Drives[DRIVEID]
.Items[ITEMID]
.Children
.Request()
.GetAsync();
var uploadSession = await _graph.Drives[DRIVEID]
.Items[ITEMID].ItemWithPath("test.docx").CreateUploadSession()
.Request().PostAsync();
var maxChunkSize = 320 * 1024; // 320 KB - Change this to your chunk size. 5MB is the default.
var provider = new ChunkedUploadProvider(uploadSession, _graph, fileStream, maxChunkSize);
// Setup the chunk request necessities
var chunkRequests = provider.GetUploadChunkRequests();
var readBuffer = new byte[maxChunkSize];
var trackedExceptions = new List<Exception>();
DriveItem itemResult = null;
//upload the chunks
foreach (var request in chunkRequests)
{
// Do your updates here: update progress bar, etc.
// ...
// Send chunk request
var result = await provider.GetChunkRequestResponseAsync(request, readBuffer, trackedExceptions);
if (result.UploadSucceeded)
{
itemResult = result.ItemResponse;
}
}
// Check that upload succeeded
if (itemResult == null)
{
// Retry the upload
// ...
}
return "Success";
}
}
catch (ServiceException e)
{
// Debug.WriteLine("We could not upload the file: " + e.Error.Message);
return null;
}
}
As soon as the code hits:
var result = await provider.GetChunkRequestResponseAsync(request, readBuffer, trackedExceptions);
trying to request:
"https://myapp.sharepoint.com:443/sites/dev/_api/v2.0/drives/DRIVEID/items/ITEMID/uploadSession"
is thrown.
What and where do I have to apply required permissions? The Documentation refers to Files.ReadWrite Does Sharepoint need additional permissions to Microsoft Graph API?
Within the shared folder I can see a new created tmp file with 0 bytes.