I’m baffled here. I tried the GitHub version of the code - checked that the application in Auth0 was registered as a regular web app, and it worked first time. I then stopped debugging, restarted the app and tried again after logging out of the web app first. Same issue. There is practically nothing different between the two solutions (yours and mine) other than the Auth0 application config settings.
I’m running these in VSCode on Win10, testing on Firefox and Opera under private browsing.
Signing up for the first time works fine every time. Logging out and signing in again with those same credentials causes the issue.