Windows EC2: SSL Connection could not be established

Verify SSL/TLS Configuration: Certificates: Check if any certificates required for SSL/TLS connections have become corrupted or are missing. This includes the root certificates used to verify SSL connections.

TLS Settings: Ensure that the appropriate TLS protocols are enabled. The upgrade might have altered the settings. You can check and adjust TLS settings using the registry or tools like IIS Crypto.

2. Network Security and Firewall: Security Groups and Network ACLs: Confirm that the security groups and network ACLs associated with your EC2 instance allow outbound traffic over HTTPS (port 443).

Windows Firewall: Check the Windows Firewall settings on the EC2 instance to ensure it's not blocking outbound HTTPS traffic. 3. System Updates and Patches: Windows Updates: Make sure the instance has the latest Windows updates and security patches installed. Sometimes, an update can inadvertently alter network configurations or SSL/TLS settings. .NET Framework: Ensure that the .NET Framework is up to date. Updates to the framework can fix underlying issues related to SSL/TLS.

4. Check Task Scheduler Configuration: Permissions: Ensure that the task is running with the correct permissions. Sometimes changes in storage or configuration can affect the user permissions or the environment in which the task is executed. Environment Variables: Make sure that environment variables are correctly set, especially those related to networking and SSL/TLS.

5. DNS and Hostname Resolution: DNS Configuration: Verify that the DNS settings are correct and that the instance can resolve the hostname of the API endpoint. Incorrect DNS settings can prevent SSL connections from being established.

6. Logging and Diagnostics: Event Viewer: Check the Windows Event Viewer for any related errors or warnings that might provide more insight into what is causing the SSL connection issue. C# .NET Service Logs: Enable verbose logging in your C# .NET service to get more detailed error messages about the SSL connection failure.

7. Testing Connectivity: Telnet: The failure to connect using Telnet indicates that there might be a deeper network or configuration issue. Although Telnet doesn’t support SSL, it still helps confirm basic connectivity issues. Use tools like curl or openssl to test SSL connections directly from the EC2 instance.

8. Reboot and Retry: If all else fails, consider rebooting the EC2 instance. Sometimes a restart can resolve residual issues that are hard to track down.