Can you try chrome on another PC? I had a similar issue with chrome (sitemap worked OK on firefox/IE but not chrome) which was only resolved by removing and re-installing chrome.
May be, not really sure. I completely removed chrome (also on my windows 10 PC) and then rebooted before reinstalling chrome and it all came right. I’m logged into google, so AFAIK all the same extensions loaded.
The issue with the sitemaps frustrated me for a couple of weeks, and I tried lots of different things.
Since I reloaded Chrome, the problem has not come back.
If you find the problem, I would be very interested in hearing about it.
For me the basic sitemap always worked, it was only the classic which didn’t. Out of interest if you load the demo application, does this work correctly?