It seems that Manjaro’s 5.0.0-1 version of the
yubikey-manager crashes due to inscrutable errors, with the effect of not letting me use it to sign in at all the sites I’d set up to use TOTP authentication.
If the previous version (4.0.9-1) were still in the pacman cache, then downgrading would be straightforward:
sudo pacman -U /var/cache/pacman/pkg/firefox-64.0.2-1-x86_64.pkg.tar.xz
Regrettably, I had recently cleaned things up and flushed the cache, so I had to fetch the package (and its signature) from the “y” directory of the Arch archive, then install it:
sudo pacman -U /tmp/yubikey-manager-4.0.9-1-any.pkg.tar.zst loading packages... warning: downgrading package yubikey-manager (5.0.0-1 => 4.0.9-1) resolving dependencies... looking for conflicting packages... Packages (1) yubikey-manager-4.0.9-1 Total Installed Size: 1.11 MiB Net Upgrade Size: -0.13 MiB :: Proceed with installation? [Y/n] Y <<< snippage >>>
Whereupon It Just Worked™ again.
I expect someone more experienced than I will have long since filed a bug report / sent a pull request / whatever, because I have little idea how to do any of that. The next upgrade should work just fine.