Thanks to everyone for their ideas and suggestions! I thought I’d share the denouement for anyone who might be following along.
As JMichaelTX suggested, I tried toggling Safari’s Develop → Allow JavaScript from Apple Events setting. For whatever reason, that didn’t seem to change anything.
Then, as a shot in the dark, I reset my MacBook Pro’s PRAM and its SMC, and I rebooted the machine. And now, voilà—everything is back to working again. ¯_(ツ)_/¯
Thanks again for all the ideas. I’m not sure what may have been the cause, but I’m just happen to have Keyboard Maestro back in good health again :).
— Ashley