My MERCHANT_ID is: 58V2NEF27DY7E

I downloaded the latest CheckoutNETSDK and replaced environment() method body with my Sandbox client_id and secret obtained directly from my PayPal developer account page so that it didn't get them from the environment. I also refreshed the secret several times.

I then ran the RunAllCaptureIntentFlow Sample

but I get the error:

System.AggregateException: 'One or more errors occurred.
({"error":"invalid_client","error_description":"Client Authentication failed"})'

I traced execution through to the method: PayPalHttpClient AuthorizationInjector fetchAccessToken() where it failed on the return statement:

AccessTokenRequest request = new AccessTokenRequest(environment,   refreshToken);
this.client.Execute(request);
return executeTask.Result;

The request had the following content:

{Method: POST, 
RequestUri: 'https://api.sandbox.paypal.com/v1/oauth2/token', 
Version: 2.0, 
Content: System.Net.Http.FormUrlEncodedContent, 
Headers:{  
    Authorization: Basic QWVNM1E2R25HTyAtIGNPcFJZcmJURTFBTnBNOTBCQmFOV3U2b25tM0p6bDduaHQ3TlJsQVRwY0czQ2FLOG44WVdCMWdRNWZxQkliekxBYmxKUjpFSGlBNXROZ0F2M0FxRFFfajFpUF9mWFVEemZtX3lWOWM3dlBSb1MtUm9QNFUxMHFETHNYZjR5UGg2Mkpqc3VIMlJOZVp5UjN1ZnNjcmlaNP==  
    Accept-Encoding: gzip  
    User-Agent: PayPalSDK/PayPal-NET-SDK  
    User-Agent: 2.0.0-rc2  
    User-Agent: (
        lang=DOTNET;
        v=4.0.30319.42000;
        clr=4.0.30319.42000;
        bit=64;
        os=Microsoft Windows NT 6.2.9200.0)  
    Content-Type: application/x-www-form-urlencoded  
    Content-Length: 29
    }
}

The details of the error are:

System.AggregateException
HResult=0x80131500
Message=One or more errors occurred. ({"error":"invalid_client","error_description":"Client Authentication failed"})
Source=System.Private.CoreLib
StackTrace:
at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
at PayPalCheckoutSdk.Core.PayPalHttpClient.AuthorizationInjector.fetchAccessToken() in C:\Users\howar_000\Documents\Visual Studio 2017\Projects\Checkout-NET-SDK-master\PayPalCheckoutSdk\Core\PaypalHttpClient.cs:line 62
at PayPalCheckoutSdk.Core.PayPalHttpClient.AuthorizationInjector.Inject(HttpRequest request) in C:\Users\howar_000\Documents\Visual Studio 2017\Projects\Checkout-NET-SDK-master\PayPalCheckoutSdk\Core\PaypalHttpClient.cs:line 51
at BraintreeHttp.HttpClient.<Execute>d__10.MoveNext()

Inner Exception 1:
HttpException: {"error":"invalid_client","error_description":"Client Authentication failed"}

N.B. I have edited the Authorization so no one can use my account

Any ideas why my client_id would be failing authentication?