A Microsoft file hosting and synchronization service.
Quick update to my post above: I can now upload files to shared folders again in the OneDrive for Android app.
Also, on my wife's Windows 10 tablet her shared folders are again accessible as locally synced folders, so it looks like her account and the account sharing the folders are now on the same platform. I'm still waiting for my shared folders to revert from links back to synced folders. Hope to see that happen soon, now that the Android app is already working again.