service vz start
Starting OpenVZ: failed to load module vzmon               [FAILED]

 vzmon: Unknown symbol ve_snmp_proc_init
 vzmon: Unknown symbol addrconf_sysctl_free
 vzmon: Unknown symbol ve_ndisc_init
 vzmon: Unknown symbol addrconf_ifdown
 vzmon: Unknown symbol ip6_frag_cleanup
 vzmon: Unknown symbol fini_ve_route6
 vzmon: Unknown symbol addrconf_sysctl_init
 vzmon: Unknown symbol ve_snmp_proc_fini
 vzmon: Unknown symbol ve_ndisc_fini
 vzmon: Unknown symbol init_ve_route6
 vzmon: Unknown symbol addrconf_sysctl_fini

OpenVZ "failed to load module vzmon" solution

The cause of the error is simply lack of ipv6 kernel support, it means that the ipv6 kernel module is not getting loaded or is blocked/disabled in something like modprobe.conf.

make sure the ipv6 kernel module is getting loaded (I had mine disabled in /etc/modprobe.conf).