I think i found the solution :
TLS 1.2 and .NET Support: How to Avoid Connection Errors - Perficient Blogs
//in .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;
ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;
// ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls;
So it was a .NET problem after all. Hmmm.
I am leaving this here with the result i find, maybe it will help others too.