Just as a heads up,
https://www.mobileread.com/forums/sho....php?p=2606586 reports a problem if you do an "Add to library" from the device list. It looks like it only happens with sideloaded kepubs. I don't think it is directly related to the driver as when I renamed an epub to be a ".kepub.epub", it failed as well. I'm going to have a look at what is happening, but it is an area of calibre that I haven't really looked at before.
The other thing that this brings up is that adding a sideloaded kepub from the device should add the book as a kepub. At the moment, I think calibre is trying to add it back as a epub.
Edit: Looks like it is something in the main driver. It is part of the pre-Touch driver. I'll have to work out what to do with it.