It's providing thumbnails via the newer IThumbnailProvider method which was added in Windows Vista. Opus doesn't support that method yet, unfortunately.
(Until now it hasn't seemed worth supporting IThumbnailProvider as very few things have used it without also providing thumbnails via the older method which Opus does support.)
I'm in the same position as the original poster. Are there any plans on supporting IThumbnailProvider methods currently?
Is that still the case? What about installing the 32bit distribution of Adobe Reader? ..but perhaps the 32bit libraries wouldn't plug into 64bit DOpus, then.
The old (pre-Vista) style of thumbnail provider runs in-process so it has to be the same "bitness" as the process requesting the thumbnails. Same reason that Adobe Reader thumbnails don't appear in Explorer on 64-bit machines.
A 32-to-64 bridge could be written to make it work (like the one I wrote to allow viewing PDF files in 64-bit Opus before I discovered how to fix Adobe's preview handler for 64-bit) but it's quite a lot of work just to make one thumbnail format work again when the fault is really on Adobe's side. It's something I might look into in the future if time permits, though.
You're actually right it's all Adobe Fault. I really don't know how much time it takes to write a 32to64 bridge in the register. But if the final solution is as "simple" as the preview fix you did it definitely worth the price to try it. You will be a hero for all PDF user who needs thumbnails to manage large amount of PDF files.