Apache2 not receiving http / port 80 traffic

Due to reasons I cannot fathom, my server stopped passing port 80 traffic to apache2.
After many hours of troubleshooting, I eventually discovered that it was only listening on IPv6 traffic and not IPv4. I have no explanation for this spontaneous regression, but the fix was simple.
# sudo nano /etc/sysctl.conf
Add net.ipv6.bindv6only=0
# sudo reboot

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