After configuring an ebook device and pressing "Finish":

calibre, version 3.20.0
ERROR: Startup error: There was an error during calibre startup. Parts of calibre may not function. Click Show details to learn more.

Traceback (most recent call last):
File "/usr/lib/calibre/calibre/gui2/main.py", line 300, in initialize_db_stage2
self.start_gui(db)
File "/usr/lib/calibre/calibre/gui2/main.py", line 238, in start_gui
main.initialize(self.library_path, db, self.listener, self.actions)
File "/usr/lib/calibre/calibre/gui2/ui.py", line 255, in initialize
LayoutMixin.init_layout_mixin(self)
File "/usr/lib/calibre/calibre/gui2/init.py", line 566, in init_layout_mixin
self.vl_tabs = VLTabs(self)
File "/usr/lib/calibre/calibre/gui2/init.py", line 402, in init
self.setTabsClosable(gprefs['vl_tabs_closable'])
File "/usr/lib/calibre/calibre/utils/config.py", line 420, in getitem
return self.defaults[key]
KeyError: 'vl_tabs_closable'