Anonymous save not allowed error thrown by SaveMediaAsync

We are having issues with uploading assets using the SDK.  
The call to `SaveMediaAsync` always results in an exception (result from the API) with the message `Anonymous save is not allowed` (statuscode 403).

When we follow the same steps, with the same data, but using Postman, the upload succeeds. 
Only when we try it via .NET it fails.

Let me know if you need help reproducing the error.