[Kobo] Some books 'are virtual and cannot be added...'

I have Obok and the Extended plugins running. I forgot some old books on the Kobo and found them by accident. They are on the device but will not load into the calibre library. One example:

calibre, version 5.22.1
Not Implemented: The following books are virtual and cannot be added to the calibre library:

E:\.kobo\kepub\05288efe-a18e-441d-9e62-fbdc2309c8c1

What am I doing wrong? Other kepubs, in the same folder, work fine in the calibre library.

Cheers,

Chris