Problems with vsftpd

[Alessio]

Hello, I've installed on my RT-ac66u this firmware 3.0.0.4.372.31 (Merlin build).
I have a problem with vsftpd: I have a server with CPanel installed, and CPanel is not able to transfer my daily backup using a ftp connection.

The strange thing is that if I try to connect from shell I'm able to ftp connect to my local nas, Cpanel have also a utility wich tests all the backup destinations, and this utyility reports that it is able to connect to my local nas, but every night when chron starts the backup process, I receive this message

+===================================+
| Transport Disabled |
+===================================+
| Name: xxxxxxxxx
| Type: FTP
| Remote Host: xxxxxxxx
|
| Reason: Transport surpassed error threshhold with “21” errors
+===================================+

I've installed entware, I've mounted the jffs partition and now I'm trying to configure \jffs\configs\vsftpd.conf
here is my configuration:

# Standalone mode
listen=YES
max_clients=0
max_per_ip=0
# Access rights
anonymous_enable=NO
local_enable=YES
write_enable=YES
nopriv_user=root
anon_upload_enable=NO
anon_mkdir_write_enable=NO
anon_other_write_enable=NO
# Security
anon_world_readable_only=YES
connect_from_port_20=YES
hide_ids=YES
pasv_min_port=50000
pasv_max_port=60000
# Features
xferlog_enable=NO
syslog_enable=NO
ls_recurse_enable=NO
ascii_download_enable=NO
async_abor_enable=YES
# Performance
one_process_model=NO
idle_session_timeout=120
data_connection_timeout=300
accept_timeout=60
connect_timeout=60
anon_max_rate=50000
# rt-ac
chroot_local_user=YES
local_umask=000
dirmessage_enable=YES
use_sendfile=NO
pasv_enable=YES
ssl_enable=NO
tcp_wrappers=NO
ftp_username=anonymous
ftpd_banner=Welcome to the jungle.

Can someone please help me to configure correctly vsftpd?

Thanks in advance