Issue FTP randomly stops working, only SFTP is accessible.

[Rowan]

This wasn't the first time but ProFTPD randomly stops working while it's still listening on port 21 it doesn't accept requests other than SFTP.

Status: Connection established, waiting for welcome message...
Error: Connection closed by server

I've already tried re configuring passive FTP with the following documentation:

 How to configure the passive ports range for ProFTPd on a Plesk server behind a firewall

Applicable to: Plesk for Linux Question How to configure the passive ports range for ProFTPd on a server behind a firewall? Answer Note: When configuring the passive port range, a selected port ...

support.plesk.com

Within that documentation I also followed the firewall rules and NAT policy but all have no success.

Does anyone have any suggestions that I could follow? the symptoms started a few months ago after a Plesk update.