Actually it does not exactly crash. The first PDF file selected in a lister is previewed correctly by the PDFXchange Preview Handler. But then, docsvw64.exe seems to hang since after selecting another file in the lister (PDF or any other file), the preview pane stays empty.
Then, Dopus has to be killed via Task Manager. After a restart of Dopus, the same problem starts all over again -- only with a second (third... etc.) instance of docsvw64.exe.
Thanks already for help.
Regards David.P
What happens if you view the same sequence of files in Windows Explorer (assuming PDF-XChange's preview handler is set to be used in that as well)?
By the way, you can probably kill docsvw64.exe, then close and re-open the viewer, to avoid having to restart dopus.exe.
Actually, the PDF preview does not work at all in Windows 7 Explorer 
I guess I'll have to contact Tracker Software about that issue then...
Regards David.P
It might be worth rebooting, if you haven't tried that already, in case a component of the viewer is in a stuck state from the previous attempts. But if it still doesn't work in Explorer, definitely worth talking to Tracker to get that resolved first as it indicates something wrong with the viewer in general.