Thanks for the links.
The paths to the thumbnails in your WP-Juicebox gallery's XML file all look fine.
Here's the path (taken from the gallery's XML file) to the first thumbnail in the gallery:
https://wp-test.8014.net/wp-content/uploads/sites/1/nggallery/test-1/thumbs/thumbs_test2.jpeg
This looks to be OK. Inside the 'test-1' folder in the path is a subfolder named 'thumbs' (expected for NextGEN) and the filename for the thumbnail image itself has the prefix 'thumbs_' (also expected for NextGEN).
However, when I go directly to the location above in a browser, I get an error 404 (file not found).
Also, going directly to /wp-content/uploads/sites/1/nggallery/test-1/thumbs/ results in an error 403 (forbidden).
Please check that the file structure on your web server matches the expected structure from the paths in the gallery's XML file.
First of all, FTP into your /wp-content/uploads/sites/1/nggallery/test-1/ folder to make sure that there is actually a folder named 'thumbs' there.
Then check inside the 'thumbs' folder (if it exists) to see if you thumbnail image files are there. (They should all have the prefix 'thumbs_'.
If everything seems to be present on your web server, then check the permissions on the files and folders to make sure that they are not too restrictive.
Default permissions of 755 for folders (i.e. the 'thumbs' folder) and 644 for files (all the files inside the 'thumbs' folder) should be fine.
I hope this helps.