Thanks for the report, that was a bug in the plugin. Looks like the standalone version isn't really used by anyone so nobody noticed it.
With the latest version on Github (
d767857) this should be fixed.
If you don't have a Github account, I've attached a test version here - let me know if this works for you, I don't think the plugin has ever been tested on a Raspberry.
The latest Calibre version that runs on XP seems to be 1.48 - I can't even run that version anymore on a modern system and 2.0.0 changed lots of UI stuff around, so I don't think I'll spend the time making the Calibre plugin work with XP. The standalone version, though, should work with the bug now fixed.