Ah... The thumbnails for the Android app goes thru a special scaling server that takes the slide fotos and generates "HiDPI" thumbnails to make them look better on HiDPI screens, like modern smart phones. This was before the HiDPI settings in the client...
This server didn't support https, but it does now. So if you reload the album in the app it should work now!
Still not getting the thumbnails, but I don't know how to force the app to reload an album. I suspect I'm looking at something that's been cached, but I have no idea how to clear it.
Edit: Similarly, on my Travels album, the RSS files were mangled, and I've now updated them with the correct versions. But the app continues to refuse to open any of the folders, claiming that RSS is disabled.
Making progress. The Travels album appears to work. The AppTest album is giving me no thumbnail images, but at least there aren't any error messages in the server log, which is a good sign. I suspect something is "stuck" somewhere - I'll try creating a new one from scratch....
Yup, AppTest2 works just fine. I'll leave AppTest up for a while in case you want to play with it, but I suspect that the problem relates to the app on my own S7 "remembering" something it shouldn't.
I now did some deeper tests with AppTest and that worked too, so I'm pretty confident that it works, as long as the app itself doesn't have any cached data. So you may delete that album if you like.
So, unless someone is having issues with an existing album, I'd say this issue is fixed.
(Now that I have a Win10 desktop, a macOS laptop, and an Android phone, I can achieve the "pain in the ass trifecta!")
It's good to know what to expect if we ever create another bug.
The caching on AppTest is very stubborn. I added another image, but the app still showed 6 blank thumbnails, rather than 7. I deleted the album, and added it again, with a different title. Still no change.
The only way to get it to "let go" was to upload it to a different directory on the server. That finally did the trick.
A lesson to remember for the future - once the app gets an idea into its head, it never lets go of it!