HTTP Monitoring SSL error on Windows 2008R2

We use send-http for our Duplicati Monitoring.
Recently we replaced our monitoring server SSL Certificate with a lets encrypt one.
But now some Clients can´t connect anymore. On a debian 11 client I updated the cert store an don´t have any problems. On a Windows Server 2008R2 this didn´t help. Also i tried to install the newest canary build, but this didn´t help either.

Here is the error message from the client. Sorry that this is in german.
2023-06-20 10:36:56 +02 - [Warning-Duplicati.Library.Modules.Builtin.ReportHelper-ReportSubmitError]: Fehler beim Senden der Nachricht: System.Net.WebException: Die Anfrage wurde abgebrochen: Es konnte kein geschützter SSL/TLS-Kanal erstellt werden.

The other issue is that i can´t find this error messages. I only see them in the tray as balloon popup and in the error popups in the website to dismiss When i click on open nothing happens or i get to the protocol section but there is no error.


Let’s Encrypt will never allow to generate a cert using SHA-1. Did you install the latest ‘security’ features for 2008 R2 ?

These SHA-2 Updates alone didn´t help. Windows update was broken so i don´t know what was there or now.
I then used WSUS Offline Update to install all missing updates and now it works again.
Thanks for the help.