1) I have always used my Superhub in Modem mode. The only time it ever ran in Superhub mode was to switch it to modem mode.
2) The WAN out from my Superhub went into my main Router. From there all my home network accesses the internet.
3) Saturday PM, all devices on network suddenly lost the internet.
4) Backtracing the fault, I isolated the Superhub not accessing the internet (despite having the blue light on the top).
5) At that point I was connected into the Superhub in Modem mode directly. I was able to see the network logs and the error I posted.
6) I called VM tech support. Agent agreed I was due a new Superhub. They were also sure that for reasons they couldn't explain Modem Mode was "broken" in my Superhub. They said they could see my Superhub was online, but unable to access it.
7) Because waiting till Tuesday would be a PITA, I reset the Superhub to factory settings as a final Hail Mary.
😎In Superhub mode, could access internet.
9) Switched back and forth between Superhub mode and Modem mode. Enough to determine that I can access internet in SH mode, but not in MM
10) Because I now have a limping along home network, I'm not inclined to try out any procedures that involve long waits at the moment. I will bear them in mind if the new Superhub (which doubtless won't arrive till far too late tomorrow) looks like playing up too.
Many thanks to all for comments and suggestions.
p.s. I didn't use the MAC Clone option on my router when I set it all up those years ago. If memory serves it was needed way back when, but not with Superhubs.