[Proftpd-user] unstable performance connections hang/drop : aborting transfer: Data connection closed

Hi all, 

I'm writing in seek for help for an issue on one FTP server with Ubuntu 22 LTS and proftpd 1.3.7c just installed from scratch. 

This server is behind an Enterprise Firewall, and is being installed in place of an old Debian/vsftpd server , same public and private IP. 
No problem with the old installation that worked for years. 

The problem is that FTP clients connections are unstable, after a random time, and especially after some activity they just hang. 

I have not specified Masquerade Address at the moment, I tried just for a brief time and I got the same errors. I need connections to this server from external public IPs and internal private network 
The errors I see on profited.log are only: 

aborting transfer: Data connection closed 

I can see in real time on the FTP server ports are opening regularly as they should , at every command I make when logged in , even an "ls" open all the ports in passive "style" . 
I can reproduce the issue if I login and type for example "ls -l" manually continuously for 20 times, I can see that all ports are opening between me and the server, but after some random time is just stop with the last command and stays there locked. 

I tried check firewall settings, I cannot find the problem there , FTP is opening ports ok , so it's not a "blocking ftp active/passive issue", but I cannot understand why everything was working fine before with the old server and now it's so unstable. 

Anyone with come idea? 

Best Regards, 

Simone

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