Good day to you, gentlemen!
Joined CGTrader a couple of days ago, now facing the very same issue. Which is worrying, since the discussion is over half a year old and the problem is still in it's original state and quite persistent.
I sincerely believe this is an issue with CGTrader's servers (or those that are responsible for storing Marmoset previews for the website) and not with browsers we use or any of our local stuff.
Here's why I believe that: after re-uploading both .HTML and .MVIEW files (that Marmoset Toolbag generates when exporting a Viewer file) to CGTrader, everything loaded perfectly on all browsers on multiple laptops and smartphones, which weren't connected to each other by any means (asked a friend from another country to help XD). However, as it was previously stated, after a couple of minutes (5, maybe 10), it all goes black everywhere with the same error, which I'm sure we've all seen too many times now :) PACKAGE FILE COULD NOT BE RETRIEVED and all that jazz. After a couple of hours the Viewer file randomly succeeded to load on one phone with Samsung's internal browser (nowhere else) and then went dark again.
Maybe I'm not a 100% accurate with my little experiment, but to me it's convincing enough that it is CGTrader's side that's causing the trouble. No matter the details why it really happens, the sad truth is: *CGTrader's Marmoset Viewer feature is basically unusable because of it's inconsistency.*
Pity it's like that, I was really excited to join CGTrader partly because of the ability to embed Marmoset previews (cause Marmoset Toolbag is awesome for real-time PBR rendering). Since CGTrader's tech support hasn't solved the issue yet, I'm guessing it's either extremely complex or it's not a priority for them at the moment.
In any case, I guess a semi-reasonable thing to do at the moment would be to render-out videos of turntables in Marmoset and then upload to YouTube or Vimeo and use those as a temporary substitute. It sucks, but it's something, right? :) If anyone has a better understanding of the situation or a better alternative, please do not hesitate to share your thoughts, I believe all of CGTrader's PBR folks would be extremely grateful, myself included ;)