LetsEncrypt SSL certificates expire every 90 days.
Looks like something not quite working with the auto renewal of SSL certs for my person domains. Caught an email from LetsEncrypt warning me that various domains were about to lose their certificates, despite a cron job running on the server.
To edit the crontab (Ubuntu):
sudo crontab -e
Which now reads:
0 4 * * 1 /usr/bin/letsencrypt renew >> /var/log/le-renew.log
Updated cron to run on Mondays at 4am (using this handy cron calculator).
sudo /usr/bin/letsencrypt renew
…will manually renew any certificates that are about to expire.