IPv6 Error - "Failed to change IP address : SIOCSIFHWADDR: Device or resource busy - you may need to down the interface"

I'm trying to set up IPv6 for my domains. My ISP has provided me the following IPs:

2600:3c03::3d:5000 - 2600:3c03::3d:5fff

I went into System Settings -> Server Templates -> Default Settings -> Virtual IP address and entered the following IPv6 range:

2600:3c03::3d:5000 - 2600:3c03::3d:5500

I went to one of my virtual servers -> Server Configuration -> Change IP Address and chose "Allocate private address"

When I clicked "Change now" it said:

Updating IP addresses in virtual server familyfellowship.com Adding IPv6 address 2600:3c03::3d:5000 .. Failed to change IP address : SIOCSIFHWADDR: Device or resource busy - you may need to down the interface

I checked /etc/network/interfaces and saw this for the IPv6 section;

iface eth0 inet6 static pre-up /sbin/modprobe -q ipv6 ; /bin/true address 2600:3c03::f03c:91ff:feae:ebea netmask 64 up ifconfig eth0 inet6 add 2600:3c03::3d:5000/64

If I run "ifconfig eth0" I get:

root@durin:/home/cnasal/bin# ifconfig eth0 eth0 Link encap:Ethernet HWaddr f2:3c:91:ae:eb:ea
inet addr:50.116.62.117 Bcast:50.116.62.255 Mask:255.255.255.0 inet6 addr: 2600:3c03::f03c:91ff:feae:ebea/64 Scope:Global inet6 addr: fe80::f03c:91ff:feae:ebea/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:120661047 errors:0 dropped:0 overruns:0 frame:0 TX packets:103122956 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:22897716505 (22.8 GB) TX bytes:378539759257 (378.5 GB)

If I do "ifdown eth0; ifup eth0" and then "ifconfig eth0" I get:

root@durin:/home/cnasal/bin# ifconfig eth0 eth0 Link encap:Ethernet HWaddr f2:3c:91:ae:eb:ea
inet addr:50.116.62.117 Bcast:50.116.62.255 Mask:255.255.255.0 inet6 addr: 2600:3c03::f03c:91ff:feae:ebea/64 Scope:Global inet6 addr: fe80::f03c:91ff:feae:ebea/64 Scope:Link inet6 addr: 2600:3c03::3d:5000/64 Scope:Global UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:120662560 errors:0 dropped:0 overruns:0 frame:0 TX packets:103124274 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:22897904189 (22.8 GB) TX bytes:378540183698 (378.5 GB)

However, if I go back to Virtualmin, choose "Use private address", enter "2600:3c03::3d:5000", click "Already active" and click "Change now", I still get:

Updating IP addresses in virtual server familyfellowship.com Adding IPv6 address 2600:3c03::3d:5000 .. Failed to change IP address : SIOCSIFHWADDR: Device or resource busy - you may need to down the interface

I'm fine manually setting up the IP addresses on the server, but I can't seem to get Virtualmin to allow me to set the IPv6 for a domain.

Thanks, Carl