incorrect ftp username/password while installing new plugin

I encountered this problem “incorrect ftp username/password” while installing new plugin (UpdraftpPlus). I navigated to wp-admin dashboard -> plugin -> add new -> search for updraftplus -> install now. I was asking for my ftp server, account name, and password. I entered my absolutely correct account name and password, and clicked ftp and proceeded. I even tested the same ftp account and password on Filezilla to verify it could login to the ftp server on my server.

In case you want to know the ftp server configuration:
listen=NO
listen_ipv6=YES
anonymous_enable=NO
local_enable=YES
write_enable=YES
dirmessage_enable=YES
use_localtime=YES
xferlog_enable=YES
connect_from_port_20=YES
chroot_local_user=YES
secure_chroot_dir=/var/run/vsftpd/empty
pam_service_name=vsftpd
rsa_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
rsa_private_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
ssl_enable=YES
user_sub_token=$USER
local_root=/var/www/web-moc/wordpress
userlist_enable=YES
userlist_file=/etc/vsftpd.user_list
userlist_deny=NO
allow_writeable_chroot=NO

What is the plugin requirement on the account? Does it require the account has wordpress administrator privilege? So it was not clear why did it throw the error eventhough both the account and password were correct. Was it possible that it threw the error not because of account or password problem? Please help. I would very appreciate.

Andrew

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