Bujhm_CИ еще вопрос, а почему все это произошло? Все играло на Framework 4.0. Ведь до вчерашнего дня было все ОК.
Ну а что тут непонятного? На сервере со вчерашнего дня подняли уровень (безопасности), заботятся о клиентах наверное))

Bujhm_C,

https://blogs.perficient.com/2016/04/28/tsl-1-2-and-net-support/ .NET 4.6 and above. You don’t need to do any additional work to support TLS 1.2, it’s supported by default.
.NET 4.5. TLS 1.2 is supported, but it’s not a default protocol. You need to opt-in to use it. The following code will make TLS 1.2 default, make sure to execute it before making a connection to secured resource:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12

.NET 4.0. TLS 1.2 is not supported, but if you have .NET 4.5 (or above) installed on the system then you still can opt in for TLS 1.2 even if your application framework doesn’t support it. The only problem is that SecurityProtocolType in .NET 4.0 doesn’t have an entry for TLS1.2, so we’d have to use a numerical representation of this enum value:

ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

.NET 3.5 or below. TLS 1.2 is not supported (*) and there is no workaround. Upgrade your application to more recent version of the framework.