If mackerel-agent outputs the following log;

Failed to post metrics value (will retry): Post https://mackerel.io/api/v0/tsdb: x509: certificate signed by unknown authority

It’s possible that the CA certificate of the host on which mackerel-agent is installed is too old. Please consider updating the ca-certificates package or performing a Windows Update.

If connectivity isn't restored after updating the certificate, it is often because the OS you're using is unsupported or its settings are causing interference. In this case, we might not be able to offer support.