Ubuntu: Websites Refuse to Load Despite Active Internet Connection

Case: On Ubuntu (and derivatives), after booting up, websites suddenly fail to resolve and instant messengers fail to connect. Other devices on the LAN resolve addresses successfully.
Device browser fails with:
This site can't be reached
DNS address could not be found.
DNS_PROBE_POSSIBLE

Systemd’s DNS caches might need to be flushed:
confirmed on Ubuntu 16.04, 17.04, 18.04

sudo systemd-resolve --flush-caches

Resource

• networking – How do I clear the DNS cache? – Ask Ubuntu