So, your system is working normally most of the time? Or is it working normally all of the time, and you just get occasional extra log messages of unknown origin?
It could be informative to add %r to the log_line_prefix, so you would know where these connections are coming from.
I don't recognize the above (client?) log format. What is it from? Does it correspond exactly to the entries in the PostgreSQL log? The time stamps seem off, the wording doesn't suggest to me that they are just different ends of the same error, and the count of errors in one log doesn't agree with the count in the other (6 vs 3).
I should to update my openssl to see real error? How can i do that if i use ubuntu 18.04?
I wouldn't expect that to help. But how did you install it in the first place? Ubuntu 18.04 generally provides PostgreSQL 10, not 12, so you must have done something non-default to get where you are.
In my experience, this (poorly worded) error comes from the client initiating an ssl connection, but then not finishing the ssl negotiations. For example, PGSSLMODE is set to verify-ca or above, but the client has no root.crt file present.