I realized later that the interface had changed. I can’t see the plugin in grid view on media page, but if I change to list view I can bulk select and access the Regenerate Thumbnails function from the dropdown menu.
However it still doesn’t regenerate the images.
I changed to the fork/similar plugin Force Regenerate Thumbnails, https://wordpress.org/plugins/force-regenerate-thumbnails/, that worked for me.
Same here.
WP4.5.5 (with multisite) which should have 100% compatibility according to its author.
