Unable to verify certificate(s) found at https://api.paypal.com

Started receiving these exceptions in production for our Paypal webhook:

PayPal.PayPalException: Unable to verify the certificate(s) found at https://api.paypal.com/v1/notifications/certs/CERT-360caa42-fca2a594-8079afec
   at PayPal.CertificateManager.GetCertificatesFromUrl(String certUrl)
   at PayPal.Api.WebhookEvent.ValidateReceivedEvent(APIContext apiContext, NameValueCollection requestHeaders, String requestBody, String webhookId)

PayPal/AUHR-214.0-51787073

All our Nuget packages are up to date:

Paypal (1.9.1)
Braintree (4.11.0)

Our environment is:

Windows Server 2012 R2 Datacenter (all updates installed)
Behind a load balancer on Azure

I can't seem to see any changes we made recently that might of caused this. Does anyone have any ideas how we go about fixing this?

Edit
Downloading the certificate, and runnning the following code:

        var str = @"-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----";

        string[] strArray = str.Split(new string[2]
{
    "-----BEGIN CERTIFICATE-----",
    "-----END CERTIFICATE-----"
}, StringSplitOptions.RemoveEmptyEntries);
        foreach (string str2 in strArray)
        {
            Response.Write(str2 + "<hr>");

            string s = str2.Trim();
            if (!string.IsNullOrEmpty(s))
            {
                X509Certificate2 certificate = new X509Certificate2(Encoding.UTF8.GetBytes(s));
                if (!certificate.Verify())
                {
                    Response.Write("Unable to verify the certificate(s) found<hr>");


                    X509Chain chain = new X509Chain();

                    try
                    {
                        var chainBuilt = chain.Build(certificate);
                        Response.Write(string.Format("Chain building status: {0}", chainBuilt));

                        if (chainBuilt == false)
                            foreach (X509ChainStatus chainStatus in chain.ChainStatus)
                                Response.Write(string.Format("Chain error: {0} {1}", chainStatus.Status, chainStatus.StatusInformation));
                    }
                    catch (Exception ex)
                    {
                        Response.Write(ex.ToString());
                    }
                }
            }
        }

Returns the error:

Chain building status: FalseChain error: Revoked The certificate is revoked.

This is why the exception is being thrown. Not sure how to solve.

Solution

Work-around can be found here:
https://serverfault.com/questions/961681/revoked-ssl-certificate

Paypal's official .NET SDK latest version seems to reference a revoked certificate somewhere. Not good.