If calibre won't open it after renaming this more than likely means the file is still DRM'ed.
Maybe "Apprentice Alf" might have something for Kobo DRM?
If he does, you might also want to look at the update being done to the "Modify ePub" plugin to remove the superfluous Kobo added <span> </span> classes. See
https://www.mobileread.com/forums/sho...&postcount=637 for the newest version of the modified plugin. You will have to install this manually by downloading it and then on the calibre plugin page select "Load plugin from file" and then select the plugin you downloaded.