Map creator: This map is not downloadable due to their owners limitations

[owmapper]

I cannot create an offline map from Mapnik. This used to work and now it doesn't - I get message "This map is not downloadable due to their owners limitations". I've seen this problem reported in the past, with no obvious solution. I'm still using v 7.1.6 but I've tried the latest (7.4.25) downloaded from the website and it has the same problem. Is this fixable?

Thanks.