I didn't change any IPv6 settings with nvOC. I'm not sure what the problem might be. If you figure it out let me know, so I can apply it for the next version.
The cmd:
sudo dhclient -v -r
(might help solve the problem, but probably not)
As you guessed, it didn't help.
there is a website:
test-ipv6.comit attempts to help you identify the source of IPv6 problems; it might help, but probably will only tell you what you already know.
I tried bringing this up in Links, but it needs JavaScript to work. I'd need to move the rig closer to where I can plug in a monitor, keyboard, and mouse to see what SJWfox says...maybe tomorrow, as I also want to move nvOC off of an SD card in a USB reader stick to the mining rig's M.2 SATA SSD.
In the meantime, I've gotten enough of a workaround set up that (1) apt-get upgrade works and (2) I have an auto-switcher up and running, as mentioned in my previous post.