Unable to run Shopify Getting started app on macOS Catalina

Question

I am trying to run the Shopify Getting started app. Configuration and steps followed up to now:

1. Running on macOS Catalina 10.15.7 (don't reply with update your macOS, please) on a late 2013 MacBook Pro Retina (don't reply with buy a new Mac).
2. Created my Shopify partner account and development shop/store
3. Installed Node.js 14.19 with nvm (10.18.1 is my default and still in use for development unrelated to Shopify)
4. Installed shopify-cli 2.15.2 from Homebrew (I was using Homebrew before)
5. Ran nvm use lts/fermium
6. Ran shopify login
7. Ran shopify app create node, naming it hello_world, and as a public app
8. Created my ngrok account and the auth token
9. Ran shopify app tunnel auth with the token
10. Ran a first time shopify app serve
11. Installed the app into my development store using the URL shown by the preceding command. I saw an "Hello World"-style line (I don't remember the exact text)
12. Stopped the app with Ctrl+C
13. Rebooted computer for a security update
14. Ran nvm use lts/fermium then shopify app serve again

Now, I am unable to see again what I saw at the end of step 11.

• Inside the Shopify development store dashboard, when I go to Applications then I click the link to my app that is listed, I get a 404 error from Github Pages.
• When I point the ngrok endpoint URL in a browser on the same Mac, it tries and fails to redirect to https://undefined/admin/oauth/authorize?client_id=1f78cccba52990ef59c873bf295d0d4f&scope=write_products%2Cwrite_customers%2Cwrite_draft_orders&redirect_uri=https%3A%2F%2Fe6aa-174-92-160-71.ngrok.io%2Fauth%2Fcallback&state=091059053102325&grant_options%5B%5D=per-user . The undefined host seems very fishy. Of course, I don't have a network node named that way.
• The same happens when I point http://localhost:8081 to that same browser on the same Mac.