Adobe pdf thumbnails appear black

I am trying to add some shortcuts to some pdf files. The problem is that when I drag and drop the pdf file a black icon is placed at the dock instead of a thumbnail of the file.

If I drag and drop a video file, the thumbnail that appears in the explorer appears also in the dock. However, for pdf files I found no way to make the thumbnail appear correctly as in windows explorer. I am always getting a black icon as a shortcut.

I am using Object Dock plus v1.90a (build 538u) and windows vista sp1. I had the same problem in previous versions of object dock, that is why I tried upgrading to the beta version, but with no luck.

Has anyone experienced the same issue?

Any suggestions please?

Thanks!