I was afraid of that
What I suggest is:
- Uninstall the one you can
- Open the plugins directory. The easiest way is to go to the Miscellaneous page of the preferences and press the "Open calibre configuration directory". The plugins directory is in it.
- Close calibre
- Find the file for the plugin. It should be "KePub Output.zip". Delete this file.
- Start calibre hopefully everything is OK.