The image_absolute url referred in the Gallery chunk (galItemThumb) is set in line 113 of the Gallery snippet as:
$itemArray['image_absolute'] = $item->get('base_url').$filesUrl.$item->get('filename');
This causes an erroneous output:
/site_baseurl//site_baseurl/assets/gallery/{album_id}/{image_id}.{file_extension}
As a result, when you click the image thumb or link you get 'This image failed to load.'
To resolve this set
$itemArray['image_absolute'] = $filesUrl.$item->get('filename');
to get the correct absolute url:
/site_baseurl/assets/gallery/{album_id}/{image_id.{file_extension}
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