Could you take a look at the attached .7z archive containing config.xml, index.html and the jbcore subdirectory.
There's no file attached to your post.
I've just checked and, unfortunately, the forum software does not allow .zip or .7z files to be attached to posts. Sorry about that. (It accepts mostly image and text files.)
Perhaps you could upload your .7z file somewhere else (perhaps your own web server or a file sharing service such as Dropbox or Google Drive) and provide a download link. Thank you.
I find that the same kind of failure (all html files, no images) happens sometimes with the galleries that came with LR,as well.
This certainly suggests that the problem lies with core Lightroom functionality rather that something specific to the Juicebox web engine.
The number of images does seem to make a difference, in that if there are only a few images to test, the export works, but with a few thousand images, it fails.
If the problem happens consistently when you try to export a few thousand images, then it sounds like Lightroom might be reaching some kind of internal limit (or maybe even a system limit such as memory).
Unfortunately, it sounds like there is little that we can do with regard to the Juicebox plugin to solve your problem as it happens elsewhere, too.
Also, I notice that you are using the latest version of Lightroom CC 6 (6.14). If the problem is a bug in Lightroom CC 6.14, then there are no further upgrades for Lightroom CC 6 and the next step up would be Lightroom Classic CC 7 (but I do not know if this will help).
I've found a couple of forum thread online which might be somewhat relevant to your problem (or at least help to explain what might be going on).
The first thread is a report of Lightroom CC 6.14 maxing out system memory so I guess this might also be what is happening with your scenario.
https://feedback.photoshop.com/photosho … out-memory
The second thread is a report of Lightroom CC 6 not exporting all selected images. (There are suggestions to close Adobe Bridge and any other programs which might be monitoring the output directory.)
https://feedback.photoshop.com/photosho … ted-images
I do not know if the information contained in these threads will help but it is almost certainly a Lightroom issue (rather than an issue specific to the Juicebox plugin) and the only thing I can suggest is that you try upgrading Lightroom if you can (although I realise that there are no further upgrades for Lightroom CC 6 and the next step up would be Lightroom Classic CC 7).