You could try deselecting "Configure Layouts" from KDE's settings (but leave "Show layout indicator" selected), and create a /etc/X11/xorg.conf.d/90-keyboard.conf file:
Option "XkbLayout" "us,ca"
Option "XkbOptions" "grp:alt_shift_toggle"
as a workaround. If you already have an /etc/X11/xorg.conf file you can put the code there instead.
The error message you get sounds like a library incompatibility. Is this a vanilla full
install of Slackware? I run Slackware 14.0 64bit on my machine and a 32bit virtual machine and cannot reproduce your error.