I am having this same problem. I now have only 1 more download / month remaining for my product, so I'm reluctant to try again lest I be locked out of actually using my product. It's funny you mention the Ultimate Career Guide: 3D Artist, because that's exactly the product I'm having trouble with.
It seems when it tries to save to Dropbox, it wants to save to a Location called "Saves", but this location doesn't exist in my Dropbox space, so it just says "mkay I tried it and also I'm gonna consume one of your available downloads / month". Quite frustrating.
I opened Chrome console on that screen and I got two errors:
Our embeddable contains third-party, open source software and/or libraries.
To view them and their license terms, go to
http://goto.zendesk.com/embeddable-legal-notices
not-found
Failed to load resource: the server responded with a status of 404 (Not Found)
application-db8d1a0e10c943fc915066348b70e292.js:322 product item directive - Sscope.initPlayer
application-db8d1a0e10c943fc915066348b70e292.js:149 Uncaught
TypeError: The element or ID supplied is not valid. (videojs)
at videojs (application-db8d1a0e10c943fc915066348b70e292.js:149)
at application-db8d1a0e10c943fc915066348b70e292.js:322
Seems the page has some trouble, but I feel like this isn't really where the issue we're experiencing is.
I'll contact customer support and report back here if I have a solution.
P.S.
I just want to say that a download limit / month is a bit silly in my opinion. We bought the product, didn't we? Don't we have a right to get it from the store whenever we need/want? Moreover, problems like the ones we're experiencing complicate matters, because we're running into an issue where we aren't downloading it, yet the servers think we are, and so now we're locked out of using our products for a month.
EDIT:
Oh, sorry, should have spoken after I tried to see what would happen after consuming the limited downloads / month. All you've got to do is prove you're not a robot and you'll get the limit reset. My bad!
EDIT II:
Ah, okay. So turns out that when you go to the product's main page, there's also an option to download all the files individually, and this triggers a normal browser download, which works as expected. Still a bit strange how the Dropbox functionality seems broken.