circling back in case anyone else ever finds themselves in this situation lands on this page….

While Swiss could never confirm what the problem was (and neither website nor app worked to check in, and consistently displayed a “booking not found” error) it appears that it was related to immigration paperwork. I was traveling to another country (non-Schengen/EU) for which I was not required to have a visa for a tourist stay of up to a certain number of days. My departure leg out of that country was back on another Star Alliance carrier (not a Swiss roundtrip) AND I had not uploaded my passport into the Swiss app (under the “Profile” section) 72 hours in advance of the flight.

Of course I *did* have my profile information saved on the United app, and I like to think of myself as a somewhat savvy traveler, but could not find where to upload a copy of my passport in the Swiss app until I searched high-and-low and found it in the profile section at the check-in time.

I was never prompted to add my passport info in advance, there was no easy button/area to do so, etc.