See this Arch Linux bug (feel free to leave a comment there, too):
https://bugs.archlinux.org/task/43382
The problem is that calibre internally uses a forked and heavily modified offshoot of html5lib, but Arch Linux decided to strip it and use the incompatible upstream project.
The solution is to use either
calibre-git or
calibre-installer from the AUR.
Disclaimer: I maintain both.
calibre-git uses the
proper build instructions (no stripping out needed components), and calibre-installer is an
updater script with a systemd timer to auto-install the official binaries from
https://calibre-ebook.com/download_linux