Maybe what follows is common knowledge, but it caught me out:

Back when I first set up nvOC, I was having trouble with it accessing things via IPv6.  I worked around it by putting IPv4 addresses for things like the MiningPoolHub server in /etc/hosts.

Where I have my mining rig located, there's no network jack in the wall.  Stringing a cable across the room wasn't really a long-term option either, so I bought an Asus RP-N12, configured it as a wireless bridge, and plugged the rig into a switch and the switch into the bridge.  Should've worked, right?  Turns out there's an issue with the way traffic gets delivered over the bridged connection that keeps IPv6 from working.

I unplugged the network cable and plugged a USB WiFi dongle (more specifically, an Edimax EW-7811Un I'd originally bought for use with a Raspberry Pi) in.  Booted up, switched to a text console, fired up nmtui to configure it with my SSID and  password, and rebooted...and now it's connecting directly over WiFi instead of through the bridge.  IPv6 works like it should.

I've recently started work on a zpool auto-switch.  I put in code to connect to their server over IPv4 specifically.  Looks like I can rip that out and go with something simpler that'll use IPv6 if it's available.