But my one machine still on MAMP wouldn’t load Apache today. The error in the log was:
EAI 8)nodename nor servname provided, or not known: mod_unique_id: unable to find IPv4 address of "<my-machine-name>"
Some googling led to two possible fixes.
The first was commenting out a line in httpd.conf. Didn’t work.
The second was just adding another line in the /etc/hosts file. For the machine above, just (I guess the name of my machine had been changed by the network administrators?):
For Macs, run
sudo killall -HUP mDNSResponder
after changes to the hosts file to refresh.