I am having the same problem. My web server is running PHP Version 7.3.9. The download button is downloading an html file named juicebox.php.html. It has this content:
<head><title>Not Acceptable!</title></head><body><h1>Not Acceptable!</h1><p>An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.</p></body></html>
Here is a link to a juicebox PRO site.
http://jcisaacs.com/PHOTOS/180304-Horse-WEB/
I checked the config file; it is generating a blank linkURL for each image. I tried deleting that field entirely, did not solve the problem.