I think the problem may be that your gallery was, as you say in your video, never a Picasa Web Album.
My own test results show that the Picasa Web Album API is able to display images from Picasa Web Albums which were migrated over to Google Photos but not from new Google Photos galleries (which were not originally Picasa Web Albums).
(There may have been a short window during the transition from Picasa to Google where new galleries were able to be displayed but this no longer seems to be the case.)
The Picasa Web Album API is still included in WP-Juicebox for backwards compatibility (so that users with old Picasa Web Albums can still display their images).
Unfortunately, I do not know of a workaround for this (it seems that the Picasa Web Album API simply cannot display images from new Google Photos galleries) and you might need to use a different image source for your WP-Juicebox galleries (such as the WordPress Media Library or Flickr).
I realise that this is not going to be the solution that you were hoping for but it is probably what I would do in your position.