whenever I want to have a preview of a PDF file in Dopus the following happens:
First file OK, second file sometimes opens. Anyhow it can't be made to disappear. Instead the Dopus surface gets blurred which means break down of Dopus.
It's probably a problem with the PDF viewer you're using (which isn't part of Opus; Opus just tells it the PDF to display and asks it to display it in a window).
Do you see similar problems if you try viewing the same sequence of PDF files using WIndows Explorer and its preview pane?