Thanks. That fixed it.
Comments:
In the docs, the instructions say "POSTORIUS_TEMPLATE_BASE_URL = 'http://localhost:8000'. As a reader, you are thinking, I'm running both hyperkitty and postorius. Hyperkitty is at /archives/ and postorius is at /mailman3/ . What about the path '/mailman3/ for postorius?
The doc could mention the path even if it's to explain why it's not needed. (The reason perhaps it's not needed is root will redirect to '/mailman3/'.)This all seems very self-referential. The setting goes into settings.py which is the website for postorius itself. It is accessing itself. Given that's the case do you really need to tell Django about it's own URL? Doesn't it know it's own URL already? Could Django just contact itself without being configured?
Logging... when it can't find the templates, does it log an error anywhere?
The server had been up for a year. After I rebooted, the problem vanished, even after removing the new setting. So either it's caching the setting in Redis, or else the problem is really gone. Very strange.
Anyway, that was it. thank you.