@
Lemmi - can you open the epub in the Book Editor?
Also try giving calibre its very own private 'Temp' directory. I suggest you create a directory at the root of your system drive named 'CalibreData', and within it a subdirectory named 'Temporary' (those are names are suggestions, adjust to suit your locale and file system naming practices)
And then set the CALIBRE_TEMP_DIR environment variable to "%systemdrive%\CalibreData\Temporary". See ==>>
Environment variables
BR