TLS connection was non-properly terminated while connecting to pureFTP configured through ispconfig

i have recently configured ispconfig in ubuntu server. getting below error while connecting to ftp.

Code:

Status:    Connection established, waiting for welcome message...
Response:    220---------- Welcome to Pure-FTPd [privsep] [TLS] ----------
Response:    220-You are user number 1 of 50 allowed.
Response:    220-Local time is now 11:05. Server port: 21.
Response:    220-This is a private system - No anonymous login
Response:    220-IPv6 connections are also welcome on this server.
Response:    220 You will be disconnected after 15 minutes of inactivity.
Command:    AUTH TLS
Response:    234 AUTH TLS OK.
Status:    Initializing TLS...
Status:    Verifying certificate...
Status:    TLS connection established.
Command:    <username>
Error:    GnuTLS error -110 in gnutls_record_recv: The TLS connection was non-properly terminated.
Status:    Server did not properly shut down TLS connection
Error:    Could not read from socket: ECONNABORTED - Connection aborted
Error:    Could not connect to server