Why CockroachDB may close connection during data raed?

I have Cockroachdb on local servers and a PHP code to read data from it using PDO. When I run the script I get the error:

PDOException: Caught PDOException (500): SQLSTATE[HY000]: General error: 7 server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

Any thoughts why this happens?