The problem I'm encountering appeared identical to this one:
AppCenter complains and doesn't work to install anything
Unfortunately, the fix used there, i.e. removing and reinstalling the appcenter, did not work for me. The contents of appcenter.list, elementary.list and 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 /etc/apt/sources.list.d directory:
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.
