When I started in de debug mode and tried to install the plugin, it installed! And I could activate it as well. Hope that it works.
But maybe you learn something from the debug log:
calibre Debuglog
calibre 3.18 [64bit] embedded-python: True is64bit: True
Windows-8.1-6.3.9600-SP0 Windows ('64bit', 'WindowsPE')
('Windows', '8.1', '6.3.9600')
Python 2.7.12+
Windows: ('8.1', '6.3.9600', 'SP0', u'Multiprocessor Free')
Interface language: nl
Successfully initialized third party plugins: DeDRM (6, 5, 5) && Kobo Utilities (2, 6, 0) && Kobo Books (1, 3, 0) && KoboBooks (1, 0, 0)
calibre 3.18 [64bit] embedded-python: True is64bit: True
Windows-8.1-6.3.9600-SP0 Windows ('64bit', 'WindowsPE')
('Windows', '8.1', '6.3.9600')
Python 2.7.12+
Windows: ('8.1', '6.3.9600', 'SP0', u'Multiprocessor Free')
Interface language: nl
Successfully initialized third party plugins: DeDRM (6, 5, 5) && Kobo Utilities (2, 6, 0) && Kobo Books (1, 3, 0) && KoboBooks (1, 0, 0)
Turning on automatic hidpi scaling
devicePixelRatio: 1.0
logicalDpi: 96.0 x 96.0
physicalDpi: 92.5388994307 x 92.6756756757
Using calibre Qt style: True
Starting up...
DEBUG: 0.0 KoboUtilites::action.py - loading translations
DEBUG: 0.0 KoboUtilites::dialogs.py - loading translations
DEBUG: 0.0 KoboUtilites::action.py - loading translations
DEBUG: 2.1 No Kobo Touch, Glo or Mini appears to be connected
DEBUG: 2.1 rebuild_menus - self.supports_ratings=None, self.supports_tiles=None
DEBUG: 2.1 KoboUtilities:set_toolbar_button_tooltip - start: text='None'
DEBUG: 2.1 device_driver_name - could not load extended driver. Exception= No module named kobotouch_extended.device.driver
DEBUG: 2.1 KoboUtilities:set_toolbar_button_tooltip - setting to text='Utilities to use with Kobo ereaders
Driver: KoboTouch'
DEBUG: 2.1 device_driver_name - could not load extended driver. Exception= No module named kobotouch_extended.device.driver
Started up in 4.80 seconds with 144 books
Downloading plugin ZIP attachment:
https://code.calibre-ebook.com/plugins/211135.zip
Installing plugin: C:\Users\Wenker\AppData\Local\Temp\calibre_bmztm5\ pf