Could not create SSL/TLS secure channel after pay pal redirects to our site

Hi, We are using the paypal sandbox API for testing. After the order is executed at paypal we click on the "Return to Merchant" button which brings us back to the order completed page.

When we check the orders, the OrderIsPaid column is unchecked and we have an error on in the event log

Exception type: System.Web.HttpUnhandledException
Stack trace:
at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Message: The request was aborted: Could not create SSL/TLS secure channel.

Exception type: System.Net.WebException
Stack trace:
at System.Net.WebClient.UploadValues(Uri address, String method, NameValueCollection data)
at CMS.Ecommerce.Web.UI.CMSPayPalProvider.ConfirmPayment(NameValueCollection collection)
at CMSModules_Ecommerce_CMSPages_PayPalIPN.Page_Load(Object sender, EventArgs e)
at System.Web.UI.Control.OnLoad(EventArgs e)
at CMS.UIControls.CMSPage.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

The Event URL is /CMSModules/Ecommerce/CMSPages/PayPalIPN.aspx

We do have a certificate on the site (https)

Any ideas?

Thank you