My ISY configuration matches the Wiki instructions (localhost; http; 8080). Admin username and password match what's set in the Admin Console.
I deleted and re-added the ISY on a few prior attempts to start from scratch. Each time the Polisy automatically discovered the ISY (IoP, of course) and the problem persisted.
I think I made progress, though: I deleted the ISY again. This time, however, immediately after deleting it, I manually added it (ISYs > Add New ISY).
The Node Server running on PG3 now shows up in the Admin Console. I think I'm up and running again. Thanks!