404 AGW-404 from /restapi/v1.0/oauth/authorize

I am attempting to convert from using application wide credentials for using the RingOut api to using individual user authentication, and I am running into two particular errors:

When using the RingCentral.Net v5.8.0 (+ RingCentral.Net.AuthorizeUri v1.0.0), the following code throws a NullReferenceException inside of RestClient.Request:

using (var rc = new RestClient(config.ClientId, config.ClientSecret, RestClient.SandboxServer))

{

 var redirectUri = config.RedirectUri;

 var loginLocation = await rc.Restapi().Oauth().Authorize().Post(

  new AuthorizeRequest()

  {

   response_type = "code",

   client_id = rc.clientId,

   redirect_uri = redirectUri,

  });

}

Stack trace:

   at RingCentral.RestClient.<Request>d__31.MoveNext()

   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

   at RingCentral.RestClient.<Request>d__2.MoveNext()

   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

   at RingCentral.RestClient.<Request>d__3`1.MoveNext()

   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

   at RingCentral.RestClient.<Post>d__5`1.MoveNext()

   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

   at RingCentral.Paths.Restapi.Oauth.Authorize.Index.<Post>d__4.MoveNext()

   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

I have attempted to perform the same action using Postman, with a request looking like:

POST https://platform.devtest.ringcentral.com/restapi/v1.0/oauth/authorize

Authorization: Basic <"ClientId:ClientSecret" in Base64Url encoding>
Content-Type: application/x-www-form-urlencoded
response_type = code

client_id = <Client Id>

redirect_uri = <the same redirect uri>

But this constantly results in a HTTP 404 response:

{

 "errorCode": "AGW-404",

 "message": "Resource not found",

 "errors": u]

}