Why is there no connection when requesting the main domain?

Question

Domain Name System

There is Vesta CP. The domain is attached to it. I observe strange behavior.
When I go to a domain, for example, example.ru, the connection is not established, the browser says: "The connection was reset"
When I go to any existing subdomain of the same domain, for example, www.example.ru, the connection is established and the server returns the page.
This situation occurs when I log in from external IPs.
Inside the network everything works correctly.
I fight all day, I can not find the reason. What could be the issue here?

Answer 1

Perhaps the fact is that the system is overloaded (specific site). This can also occur due to incorrect SSL settings.