Hi,

I installed calibre:

Name            : calibre
Version         : 3.20.0-1.parabola1
Description     : Ebook management application, without nonfree decompression engine for RAR archives
Architecture    : x86_64
URL             : https://calibre-ebook.com/
Licenses        : GPL3
Groups          : None
Provides        : None
Depends On      : python2-six  python2-dateutil  python2-cssutils  python2-dukpy  python2-mechanize  podofo  libwmf  chmlib  python2-lxml  libusbx  python2-html5-parser  python2-pillow  shared-mime-info
                  python2-dnspython  python2-msgpack  python2-pyqt5  python2-psutil  icu  libmtp  python2-dbus  python2-netifaces  python2-cssselect  python2-apsw  qt5-webkit  qt5-svg  python2-regex
                  python2-pygments  mtdev  desktop-file-utils  gtk-update-icon-cache  optipng  udisks2
Optional Deps   : ipython2: to use calibre-debug [installed]
                  poppler: required for converting pdf to html [installed]
Required By     : None
Optional For    : None
Conflicts With  : None
Replaces        : None
Installed Size  : 92.95 MiB
Packager        : Andreas Grapentin <andreas@grapentin.org>
Build Date      : mar. 10 avril 2018 21:22:26 CEST
Install Date    : mar. 29 mai 2018 15:28:51 CEST
Install Reason  : Explicitly installed
Install Script  : No
Validated By    : Signature

$ uname -m
x86_64

If I launch calibre from the console or graphically, a crash GUI appears with the following crash details:

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'

After that (at least if it was started from the console), after closing the crash dialog, it cannot even be closed normally, the only way to stop it is to use kill -9.