One of my maps has gone blank

While working with other parts of the The Burning Blue Module I noticed that one of the maps (German Raids) is now blank. It is multi-zoned, and when I look at the zones, I can still see the map image using the Define Zone button.
I have tried to change the map image from png to jpg, and I have tried loading another map, but it still is blank.
There is no relevant message in the error log (Well, actually there is a frustrating “Error in expression” for the main map, but no more details, even though the audit trail is switched)

I have also tried to remove all the zones for the map, but it remains blank.

Some time ago, I had the same problem, but at that time, it was solved by deleting the image, saving, removing it from the vmod file, and reimporting it. This time I have not been that lucky.

An obvious solution would be to remove the map entirely, and rebuild it from the ground up, but there are lot of zones, and I am loath about redoing all that work.

It’s unlikely anyone can help without being able to see the module.

You are probably right. I just hoped that this was common newbie problem that someone could correct speedily.

In the meantime I have restored the module from Dropbox to a version that is definitely older than last time I saw the full image in the map: no success! So maybe this has nothing to do with the module?

Later I’ll try again on another computer.

I just wanted to check if counters could be moved normally on the blank map, and the map image has suddenly reappeared!
It is either the threat that I would try on another computer, or - more likely - that this has been influenced by my messing around with different versions of Vassal.