FTP on CWP stucks at 'Retrieving directory listing…'

I have setup CWP on CentOS 7 in Digital Ocean droplet. I have created some FTP accounts (On default PureFTP server) and trying to access FTP through FileZilla on windows. FTP server is configured with passive ports range from 30000 to 35000 (Initially it was 30000 to 50000). I have allowed same ports in IN and OUT TCP ports of CSF rules.

Normally, FTP connection works fine and there is no problem of using it.

Sometimes, while navigating through directories in server side, filezilla stucks at 'Retrieving directory listing...' for long time. Once the idle timeout of FTP server reached, it disconnects with message as server closed connection. This happens suddenly sometimes when I am working on FTP. If I disconnect all active sessions in server using FTP Manager in CWP, occasionally it works.

I have done some troubleshooting as below.

    Changed the connection from Passive to Active mode in FileZilla -It does not connect.
    Already disabled TLS encryption and using plain user name and password
    Tried in WinSCP. It worked for a while, but later it too had same problem.
    Tried disabling the firewall - Did not work even if reconnected
    When firewall was enabled, in FTP server log I found that it was blocking my login after successful login. But, I don't know the reason and also it did not work after disabling firewall
    Today I have found a message in log saying that cleartext is not supported on this server, please reconnect using TLS. Does this mean only connections with TLS are supported? But it is working even if connected without TLS sometimes. Also, I had disabled the TLS in FTP server configuration at the time of problem and it was not worked.

What could be the problem? I don't want to use SSH for file transfer as this FTP is being used by my clients. So, they should be able to connect to the server and upload files to specific directory. Also, I should disable the password based authentication for SSH if SSH is used for file transfer. The reason why I did not check other FTP servers is I would like to manage all things from the CWP portal only (Otherwise I have to manage main hosting things in portal and FTP things in command etc.)

Please help me to resolve the problem.

i have the same problem, did you solve? I try to prodftpd and result, but not support to cwp