New topic: while trying to resolve the error log above, I came across this API alert on my Square Dashboard:
There is a newer version of the Connect APIs available. Please consider upgrading.


Changing your API version may break your implementation. We recommend checking the changelog before upgrading.
and then the changelog states:


Square-Version: 2018-09-18
Adds a new Connect v2 API for inventory tracking, and updates the Customers API to support birthdays in customer profiles.
Obviously a recent change. I haven't clicked the Change Version button yet. Since I may not have even fixed the cron job issue above, I thought I should check back with you experts first for your recommendation - and it may be relevant to other ZC Square users.