I've reported this bug as https://bugs.launchpad.net/calibre/+bug/1870541. I also found the backwards-incompatible change in msgpack which is the culprit of this:
* Default value of strict_map_key is changed to True to avoid hashdos. You need to pass strict_map_key=False if you have data which contain map keys which type is not bytes or str. (from <https://pypi.org/project/msgpack/>) I can prepare a patch (both within the Guix build system and to send upstream) which fixes this, though I'm not sure what hashdos are or whether they are actually important to avoid in this case. -- Josh Holland
