Wouldn't it be because of the duplicate serverId? The approach using RowMapper I confess that I know little, I wanted to help better

public class ServerMessageRowMapper implements RowMapper<ServerMessage> {
    @Override
    public ServerMessage mapRow(ResultSet rs, int rowNum) throws SQLException {
        ServerMessage serverMessage = new ServerMessage();
        serverMessage.setId(rs.getInt("id"));
        serverMessage.setSenderId(rs.getInt("sender_id"));
        serverMessage.setServerId(rs.getInt("server_id"));
        serverMessage.setContent(rs.getString("content"));
        serverMessage.setCreatedAt(rs.getDate("created_at"));
        return serverMessage;
    }
}