VS Code extension to test network settings.
In VS Code: F1 > Network Proxy Test: Test Connection, enter the URL to test or use default. This will open a new editor and log the results.
If that shows a certificate error, read further down in the editor to get information on the certificate chain in the server's response.
Use F1 > Network Proxy Test: Show OS Certificates to check if the required root certificate is loaded from the OS.