Thanks for hinting me in the right direction. The issue was expired TLS cert. It had expired ~ 4 days ago.
I use Cloudflare, so I didn’t catch that was the issue. I should have had a 
but didn’t at the time, as I was traveling. So I just lowered CF’s SSL/TLS encryption mode from Strict to Full via mobile when the forums had ssl_browser error.
Anyway today I used the opportunity to switch to the stable branch. Edited /var/discourse/containers/app.yml to uncomment “version” and set it to stable:
Then did:
sudo ./launcher rebuild app
Retested forum cert from the blog server:
Works:
So, in summary, this is completely my goof. Sorry and thanks!
Edit: checked the cron crontab -l (its there) and did a --force renewal, which was successful, so the autorenew should work the next time. 