image miniature doesn't work

I’ve been using this plugin for ages and it never failed.
However, in this new version of WP something went off with the miniature size. The thing is that the preview thumbs are not cut into squares, as they are supposed to be, but show in original proportions, only smaller. I tried to check / uncheck the Preview size box in Settings->Mediafiles, but it doesn’t work. The miniature size is set 150 x 150, but it doesn’t work, the images still look like diminished originals, not as squares.

Any ideas?

https://www.ads-software.com/plugins/jquery-lightbox-for-native-galleries/