UPDATE: While attempting to further disambiguate the perceived difference in behavior between desktop and mobile platforms, I uninstalled the “Hubitat app” (which is little more than a glorified bookmark and browser front-end) on my Android device, then attempted to access the site using its Chrome browser. Same error condition emerged. Still cannot login. Refreshing login page yields:
“LOGIN ERROR
Account login timed out, please try logging in again.”
I surmise that the “app” had been holding onto some form of authorization key(s) from before my sabbatical, and wanted to eliminate that variable.
I’m now completely convinced this is a server-side, user-account level issue and not a device-, platform-, or browser-level client-side issue. Perhaps even a well-known Discourse framework hiccup with a known solution(?), so a-Googling I must go until help arrives.