Checking proftpd’s log:

Dec  7 12:23:12 fr01-srv proftpd[5563]: 192.168.0.1 (::ffff:192.168.0.117[::ffff:192.168.0.117]) - USER testuser: Login successful.
Dec  7 12:23:30 fr01-srv proftpd[5591]: 192.168.0.1 (::ffff:192.168.0.117[::ffff:192.168.0.117]) - USER testuser: Login successful.

However, in Filezilla things are different:

Error: Connection timed out after 20 seconds of inactivity
Error: Failed to retrieve directory listing

This issue is generally caused by a firewall restriction. Make sure your firewalls monitors FTP traffic and allows FTP-data connections. Edit iptables-config:

# nano /etc/sysconfig/iptables-config

And add/check that the proper modules are loaded:

IPTABLES_MODULES="ip_conntrack_netbios_ns ip_conntrack_ftp ip_conntrack_tftp"

Then restart the firewall and FTP should start working:

# /etc/init.d/iptables restart

Notes

The exact same frontend issue can happen if your firewall is blocking the FTP data transfers because the FTP server is using random ports. In this case make sure PassivePorts is configured in your ProFTPd configuration and the specified port range is also open in your firewall:
PassivePorts 60000 61000

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