Trying to debug a little further, I opened an anonymous session (Firefox), went to the static map maker, and it functioned properly.
The resulting URL also opens the expected map in a blank browser (first an anon browser, but then in a non-anon browser as well). (Tried another browser, still works)
Thinking perhaps a cache problem, I cleared my cache/history/all. No joy, in non-anon, it still gives same error.
Okay, let’s try Microsoft’s own browser, Edge. Again, same error, no go. Chrome… again, same error.
Tried a blank browser as a final test, and same URL no longer works, but returns the JSON error.