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MacBook Pro's Trackpad stops working when connecting an external keyboard

I'm trying to set up a new Ergodox EZ keyboard. Whenever I plug in the keyboard's via USB port, the MacBook's built-in Trackpad ceases to function. It responds to absolutely no input at all. This makes it impossible to configure the keyboard properly (I have to go through a wizard so the computer can figure out the keyboard uses an ANSI layout).

If I disconnect the keyboard temporarily, I can press next, plug the keyboard back in, and then continue with the wizard, but at the final screen I need my mouse again. So I have to disconnect → click → reconnect, at which point the wizard reappears from the beginning claiming that the keyboard could not be identified.

This is preventing me from successfully configuring the keyboard. How do I fix this?