mojave – App Store no longer updates apps

As of a couple of weeks ago, none of my App Store apps will update (2015 MBP 15″, Mojave 10.14.6). The Update button just spins for a bit, then stops. If I uninstall the app, I can reinstall it, but subsequent updates still fail.

The only abnormal Console output after I click Update appears to be:

AMSURLRequest: Failed to fetch GUID schemes from bag

I’ve signed out/in, deleted App Store caches from ~/Library/Caches and ~/Library/Containers, temp files from $TMPDIR../C/, and even reinstalled Mojave, but the issue persists.

Any other ideas? (I’m not in a position to install Catalina at present, whether that’s likely to fix it or not.)