I did the migration in March 2016. My Adobe Air is 22.0.0.153 (how to find it), but I don’t know what it was at the time.
FWIW, I had that issue as well, but my community site was much smaller. I had 3 files that could not be downloaded either. This was not, as far as I could tell, an indicator that the JSON files were corrupted. Well, that’s partly true. The JSON files that Ning creates are [bleep] [bleeping]ing weird. So most things that read JSON files will have nothing to do with them. But they import fine because Neil’s accounted for their weirdness.
I never did find out exactly what those files were but because we had all critical files backed up elsewhere, and they were so few, we decided to not try to figure this out and go ahead with the migration.
I must have broken the links to files later, but given that the site was live at this point, I just stuck with it. I’m pretty sure this is an unrelated issue to the export.