Vsftpd. 500 OOPS. Virtual users
The following error may occur on the client side: Fatal error: 500 OOPS: priv_sock_get_cmd if running an amd64 kernel with vsftpd-3.0.x.
To solve this bug you will need to add the following to your /etc/vsftpd/vsftpd.conf:
seccomp_sandbox=NO
Refer to https://bugzilla.redhat.com/show_bug.cgi?id=845980 and Gentoo Wiki
P.S. And just in case add next line to config file
allow_writeable_chroot=YES
for adequate working of virtual users.