Turns out I was affected by the last "continuity breach".
So I had to delete everything and start over. In my case this meant
$ cd urbit
$ git pull origin master
$ make clean
$ make
$ rm -rf urb/*-*
$ rm -rf frogstar
$ vere -c frogstar
This time when I ran :begin it succeeded
generating 2048-bit RSA key...
; ~doznec _Urban Republic_ is your neighbor
; ~doznec _Urban Republic_ is your neighbor
request approved
; ~zod |Tianming| is your neighbor
; ~zod |Tianming| is your neighbor
; ~doznec _Urban Republic_ is your neighbor
; ~doznec _Urban Republic_ is your neighbor
... and then more hoon files were downloaded, after which I got my new prompt:
~todbyt-nathes/try=>
Next time I'll start exploring Nock and Hoon some more ...