Thanks, Maxim.

By "Such lines in access log are caused by opening and closing a connection without sending any data in it", you are meaning that a client opens a connection and then closes the connection actively without sending any data, or that a Nginx worker process accepts a connection and then closes it actively without sending any data to the client?

In any case, is the TCP handshake completed?

I guess that SSL handshakes are already in process, because CPU resource is consumed much.

Just as you said, if the browser rejected my SSL cert, what could I do to solve this issue?

Thanks again.