405 method not allowed

The library is Dropbox.API (Official Dropbox .Net v2 SDK, Runtime version: v4.0.30319). 

Init code:

var httpClient = new HttpClient(new WebRequestHandler { ReadWriteTimeout = 10 * 1000 })
            {
                Timeout = TimeSpan.FromMinutes(120)                
            };

            try
            {
                var config = new DropboxClientConfig("Smart Kontor")
                {
                    HttpClient = httpClient
                };

                client = new DropboxClient(strAccessToken, config);
            }
            catch (HttpException ex)
            {
                MessageBox.Show("Exception reported from RPC layer");
                MessageBox.Show("    Status code: " + ex.StatusCode);
                MessageBox.Show("    Message    : " + ex.Message);
            }

Uploading code:

try
            {
                var fileContent = File.ReadAllBytes(source_filepath);

                using (var stream = new MemoryStream(fileContent))
                {
                    file_size = (double)stream.Length;
                    int numChunks = (int)Math.Ceiling(file_size / chunkSize);

                    progressBarControl1.Properties.Maximum = numChunks;

                    byte[] buffer = new byte[chunkSize];
                    string sessionId = null;

                    for (var idx = 0; idx < numChunks; idx++)
                    {
                        var byteRead = stream.Read(buffer, 0, chunkSize);
                        
                        using (MemoryStream memStream = new MemoryStream(buffer, 0, byteRead))
                        {
                            if (idx == 0)
                            {
                                var result = await client.Files.UploadSessionStartAsync(body: memStream);
                                sessionId = result.SessionId;
                            }

                            else
                            {
                                UploadSessionCursor cursor = new UploadSessionCursor(sessionId, (ulong)(chunkSize * idx));

                                if (idx == numChunks - 1)
                                {
                                    await client.Files.UploadSessionFinishAsync(cursor, new CommitInfo(folder + "/" + fileName), memStream);
                                }
                                else
                                {
                                    await client.Files.UploadSessionAppendV2Async(cursor, body: memStream);
                                }
                            }
                        }

                        progressBarControl1.PerformStep();
                    }
                }