MOBAC Prebuilt Map stopped working

I've been using BackCountry Navigator TOPO GPS for many years, usually with MOBAC prebuilt maps (.sqlite format). It's been superb.

Hoever, a while back (a year or so?) the app wanted to do some data migration and I think it was around then that one of my maps - a high res map of the South Island of New Zealand - stopped working. A low res one of the North Island continued to work. I've only just got around to trying to get everything working again.

When I switch to the South Island map all I see is a grid of "Internet Off/See layers menu" squares, exactly what I see when I'm off origin with the working map. If I select the option to centre the map at the default map centre, BCNav crashes. If I got to my current location (which within the map area), nothing changes. So it looks like BCNav is confused about the map origin. Changing zoom levels has no effect.

I downloaded the map from my PC again: nothing changed.

It was a very old map, generated with an old MOBAC, so I rebuilt a map using the latest version of MOBAC and the latest maps. Downloaded it to BCNav - same result.

BCNav works fine with on-line maps and the old MOBAC .sqlite low resolution North Island map. I'm using the latest version of BCNav

Any ideas, anyone?