The situation is familiar to many website administrators. The main domain opens normally: the lock icon is visible in the browser, HTTPS works, there are no warnings. But as soon as you switch to a subdomain – for example blog.example.com or mail.example.com – the browser suddenly shows a message about an unsafe connection. For the user this looks like a website error. In reality, in most cases the server itself works normally. The issue is usually hidden in the way the certificate was issued or connected.