A Microsoft file hosting and synchronization service.
Finally - I have been searching for weeks for evidence that ANYONE else on the planet has been having this same issue. Worked fine for YEARS with no issues, now, suddenly, not so much. I'll add Mozilla Firefox to the list of browsers that this is happening with. Strangely enough, it seems to be intermittent with Edge - sometimes it's fine, sometimes it gives the same "1/1" in a folder with many photos . . . and sometimes I get a different viewer that pops up and seems to overlay the webpage that allows scrolling through images.
Not sure wth is going on, but the most depressing thing you can hear from a Microsoft Agent is "report this directly to Microsoft through the Help & Feedback center. Best wishes".
Awesome. You may as well send an email to Santa Claus.