The problem I'm encountering appeared identical to this one:
Unfortunately, the fix used there, i.e. removing and reinstalling the appcenter, did not work for me. The contents of
patches.list are all identical to those in the above-cited question. Since the "details" says it's an authentication problem, I doubt this is due to a problematic repository, but for the record here are the files in my own
appcenter.list appcenter.list.save elementary.list elementary.list.save eugenesan-ubuntu-ppa-bionic.list fingerprint-ubuntu-fingerprint-gui-bionic.list fingerprint-ubuntu-fingerprint-gui-bionic.list.save patches.list patches.list.save philip_scott-ubuntu-elementary-tweaks-bionic.list philip_scott-ubuntu-elementary-tweaks-bionic.list.save phoerious-ubuntu-keepassxc-bionic.list phoerious-ubuntu-keepassxc-bionic.list.save
All help will be greatly appreciated.