There are a few reasons for this, but this is commonly fixed by adding the follow code to the application start event:

if (ServicePointManager.SecurityProtocol.HasFlag(SecurityProtocolType.Tls12) == false)
        {
            ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12;
        }

The way of registering application started/starting events has changed in V8 though.

See here:

https://our.umbraco.com/documentation/reference/events/application-startup

So I think you should add the following to your own compositions class:

using System.Net;
using Umbraco.Core.Composing;

namespace YourNameSpace.Compositions
{
    public class ApplicationComposer : ComponentComposer<ApplicationComponent>, IUserComposer
    {
        public override void Compose(Composition composition)
        {
            // ApplicationStarting event equivalent: add IContentFinders, register custom services and more here
            base.Compose(composition);
        }
    }

    public class ApplicationComponent : IComponent
    {
        public void Initialize()
        {
            if (ServicePointManager.SecurityProtocol.HasFlag(SecurityProtocolType.Tls12) == false)
            {
                ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12;
            }
        }

        public void Terminate()
        { }
    }
}

Hi Matt,

Thank you so much for your input. I added the suggested class and also tried several possibilities from the other article, but unfortunately without success. The error continues to appear in the log at the first time I arrive at the Umbraco Welcome dashboard after opening the backend in a browser. It seems that the error does indeed only occur after Umbraco has restarted, for example by an app pool reset or a change in code / web.config.

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