@3Dscrewer @JimmyKater thank you. Let me provide what details I can. I am using a freshly made SD card (with the nextcloudpi image dated 6 Feb 2018). My nextcloud box is connected by ethernet to my router and, with the card in the pi I can see it (and its ip 192.168.0.xx) by logging in to the admin from my router/wireless box. Booting the pi whilst attached to the TV all seems fine. At the prompt I can login using the standard pi credentials (pi/raspberry) and then I can run sudo nextcloudpi-config and a menu appears. Any attempt to do anything on the menu gives error 126.
I have tried all possible ways of using a laptop connected to the wireless box to connect to the pi (e.g. https://192.168.0.xx:4443) but to no avail. The laptop is running Ubuntu.
With grateful thanks (and sorry to bother you)!