Estoy implementando una tienda en línea durante los últimos meses y la he conectado con éxito a Sandbox of paypal para la entrada de PayPal pro pro gateway. Funcionó perfectamente desde el principio.

Desde el fin de semana ya no funciona. La tienda me da el siguiente error:

    ERROR CALLING PAYMENT GATEWAY

    The trace gives me this error:

    Could not create SSL/TLS secure channel

    Page URL:/checkoutreview.aspx  Source:System.Web.Services  Message:The request was aborted: Could not create SSL/TLS secure channel.
  Stack Trace:
     at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request)
     at System.Web.Services.Protocols.HttpWebClientProtocol.GetWebResponse(WebRequest request)
     at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
     at AspDotNetStorefrontGateways.Processors.PayPalAPIAASoapBinding.DoDirectPayment(DoDirectPaymentReq DoDirectPaymentReq) in C:\Development\Natrol\AspDotNetStorefront\ASPDNSFGateways\PayPalSvcAPIv30.cs:line 956
     at AspDotNetStorefrontGateways.Processors.PayPal.ProcessCard(Int32 OrderNumber, Int32 CustomerID, Decimal OrderTotal, Boolean useLiveTransactions, TransactionModeEnum TransactionMode, Address UseBillingAddress, String CardExtraCode, Address UseShippingAddress, String CAVV, String ECI, String XID, String& AVSResult, String& AuthorizationResult, String& AuthorizationCode, String& AuthorizationTransID, String& TransactionCommandOut, String& TransactionResponse) in C:\Development\Natrol\AspDotNetStorefront\ASPDNSFGatewayProcessors\GatewayPayPal\PayPal.cs:line 415
     at AspDotNetStorefrontGateways.GatewayTransaction.CallGateway(String gateway) in C:\Development\Natrol\AspDotNetStorefront\ASPDNSFGateways\GatewayTransaction.cs:line 205
     at AspDotNetStorefrontGateways.GatewayTransaction.Process() in C:\Development\Natrol\AspDotNetStorefront\ASPDNSFGateways\GatewayTransaction.cs:line 176 

Que esta pasando aqui ? ¿Alguna idea de qué pasó y cómo resolverlo? ¿Por qué se rompería de repente?

gracias michael