I was using Debian. If you are on Ubuntu, check if you have ModemManager installed, it has a tendency of jumping on each and every serial port, which can easily result in them getting busy. The Kaleidoscope repo has some udev rules you can install, which will prevent ModemManager stealing the keyboard’s serial port.
Stick that file in /etc/udev/rules.d, and then sudo udevadm control -R should reload the rules. Plugging the keyboard in afterwards should then work.