GroupshareKit - upload project template does not work

I am trying to upload a project template to a Groupshare 2017 SR1 server using the Groupshare Kit.
Groupshare Kit version is the currently released one obtained through NuGets.
 
My code looks like this:
     Try

            Dim TemplateFileName As String = "C:\Users\xtomx\Documents\Studio 2017\Project Templates\Default.sdltpl"

            Dim GSClient = Await GroupShareClient.AuthenticateClient("Test", "Test_01", ServerURI, GroupShareClient.AllScopes)

            Dim Buffer = My.Computer.FileSystem.ReadAllBytes(TemplateFileName)

            Dim UploadTemplate = Await GSClient.Project.UploadProjectTemplate(templateId:="7f976661-4186-41c7-8aba-dabcb4ef879b", projectTemplate:=Buffer, templateName:="TESTTPL")

        Catch ex As Exception
               MsgBox(ex.ToString)
        End Try
 
The Request looks like this:
 
{Method: POST, RequestUri: 'winserv2016/.../upload', Version: 1.1, Content: System.Net.Http.MultipartFormDataContent, Headers:
{
User-Agent: (WindowsRT 8+; unknown; de-DE; GroupShare kit)
Authorization: Bearer AAIAACgvIyBHGKuvGZP8CxzNBUl3vlVYdxSrSObYQNr1Y5_xEtmpxNyKzVTpK-1kFNA7cUnuIWiLkwSPssDxLDZ_s-iuXQoTTW5BTj5aEEv0vDSvKkzFjashDHqI-yF2F5tBF06dD-b9InxKAfP_fuLJWsO6tCWo-ji0ITs9GvOkEUmZaovUctg8KAuUNUs8VXpw6MmH_WWk-9mHwweHQK21g1_2QtYGbNsc2qVA2QYq0daw5oK62SL0ZWGALhdc5kLp1UP4YjRQlL8agEh7iSI2ps-qz7vvsxcPVJfX0mfJFJP8xY09mYieSJZxuGHC5ifL652s8EZfVdKtMJVQSV7V5dpgioKLomEglK2CnElaK30z1HC2MD1I1DAa_8sMY8FUm07oUH70_C30qSMJ68zIE84ujAWu8pLtgmgf0E1LftiRChoGhVTV_NidPYTZY33_T8IR5MvAm18b_PSjfEXAu0s6eVtaifVWT1k1jcukPq0FW4boRDxViQcBEXfOzGquhx6eFgGoFM_PpP5oTuVhMG1yUUYHgvps7uuhUyu3sHQXfm_1ziLvgj5HzmXRYdaH0oT9KuQisB5Ubf6LVY_0XUrAFWOdwZ4hMe6Z-4EK9KGBKv7cC_QQC9yPvJDhZgtFOuPbyMrAXL9mKRzYEqq2Z5kSSM9eOYTAR20o5_8TOjH_xAIAAAACAABp4IA0k8KVj9Anq3otRK4xrHB4LAEgSuHTYkS1UkmghVh3-N6XpJB--4rSzjHLUPpSFp2QjAXjglL-jA8MHQ5IWBRNWNVOkJVuMaTksdE-WP5ZriGgHeWv8avm5nOgv3_uWRVlMqdwhw4xqZAW2Sb6sB_o8XUbN5AeHSuJn2TbDZ-TrtB8SR4nPHq63FW_8__BpBdmNOrciCd56Lccjny9cx8w9bzdypto0Vmnln7HBKrgD3v1fzM9F3_Q3qS4zGDvap9y0VWKpMzTnOqqibZjK4PTbo7mDCVr7LrOug5p6YY46inEukP9exV1NHSX3ywu2aJyNcimBSeNmivYGpDcCO6T8q4Y7gK6izuydb9U90zyZRasJEJelzSkaApX1PEjOejnbKzwv4lr_RySsJhwYzAmNPIvqjtTKrO8CP8wUlNCQKS31uNLwkuHplik2uSGdGoVjjxYg9XJ5BwbWn0bDHDh2P1DJqkcy-NxkO2wwHEj8Zkbfdyw_kRSdxFthCgCc7L9u4uQfzyNyU4F8yqV8lpMMpnc2ORH1VfmUmG4T6IIqqcj4XiIraNLUz7KzvYj84yXOkdkuehcGKnxpSKiwDZzRS3I3Af5cHrSUhtX0v0thSzD2ZlNyslRtDgMcE6a0GPY8WHZctputFi4GzKN-VfDN7ppLKvCFhCc0XtcWcY40sacZbCPhfayceKHtrnuYtypFniJDkacADNbiAjjfy73t-b6NnIfSt0ZcHF7mUdmaOXg5NxGyfRQKLgBG3GiLY6ovMNeNmN6d-qdnenin2ykCOYrLifwXPT-GYwwYaOgtBcD7xVvtQ5138MRpLz_fc8mPAurl2qkxdXce0S3TcBuUOccL0Ga3FEn5Uv3oZrgTyujmVd9dEJTvjpQKbaeyjMXIKFL_siAvWyQxxXCxsp6Vu0NU5SUb2PJJn3HoQ
Accept: application/json
Content-Type: multipart/form-data; boundary="423fd500-f393-4904-b045-9645d3d157cd"
Content-Length: 4745
}}
 
Response:
{StatusCode: 200, ReasonPhrase: 'OK', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
Server: Microsoft-IIS/10.0
X-Powered-By: ARR/3.0
X-Powered-By: ASP.NET
Date: Fri, 11 May 2018 10:23:35 GMT
Content-Length: 0
}}
 
However, when my function returns, the template on the Groupshare server is not created. What am I doing wrong?
 
many thanks,
Tom