The problem is fixed, although I don't know the process behind it. I was using this address to connect the browser to the calibre server (which had previously worked):
http://192.168.1.30:8080/mobile
Somewhere in the process I did get connected with an ePub reader on my phone, and I noticed that the address was:
http://192.168.1.30:8080/mobile?library_id=Calibre1
I typed this into the address field on both the Kindle Voyage and the Kobo Libra 2 browsers and suddenly each could display all my books and I could download all the latest ones.
I have no idea why one works and the other does not, but now I know the cure.
Thanks.