Solved. False alarm.
Local stupidity (on two fronts), not an Obihai problem.
1. Earlier in the day I had started toying with a pocket router operating as a WiFi repeater and had left it running while I was outside re-laying some bricks in a garden wall and then forgot about it. It defaulted to having the same IP address as my main router. I run my 110 with manually assigned IP parameters. Looks like the 110 was trying to use the pocket router as its gateway for off-subnet destinations. Soon as I powered off the pocket router, everything was OK. But strange that only the 110 was affected - my tablet was OK, my wife has been using a PC browsing the net for several hours without problem and the OBion softphone was OK too.
2. On reflection it is very probable that the OBi100 in Japan was always OK. I was viewing the Obitalk portal on a Samsung Galaxy Tab 2 7.0 and the screen on that renders the green on-line status dots almost the same as the grey off-line ones. I hadn't done any further checks that the OBi100 was really off-line to the portal.
If it hadn't been for the portal really being down in an apparently similar manner last week I would probably have done more exhaustive local investigation before posting.