Black screen with blinking cursor for 60 seconds on reboot, before windows starts –solved, Ethernet service setting.

Totally empirical approach, “by intuition and sense of smell” (one for the Who fans)
Googling “Blackscreen blinking cursor 60 seconds on reboot” gave lots of replies, with two major solutions
(i) turn off fast startup in power options (ii) disable one display adaptor if two are present in device manager.
Neither of these worked for me (i) this option not available on my setup, typed in powercfg /h off just in case (ii) only one display adaptor present.

So, went to event viewer, to search for errors consistent with appearance of problem since latest windows update (windows 10 v 1703 update 2017-07), and found

Event131 DeviceSetupManager
Metadata staging failed, result=0x80070490 for container '{00000000-0000-0000-FFFF-FFFFFFFFFFFF}'

Googled in the latter, and after trial and error, eventually found this at (thanks Jusup)

(in services) “using LAN/Ethernet, set wired autoconfig service to automatic and start the service.”

Problem gone.
Did the latest windows update or my vpn change Network Connections to “authentication” ?? Don't know.

N.B. this problem was only present on reboot, not cold start.