Opus does not generate PDF thumbnails. If a PDF viewer with a thumbnailer is installed, Opus will ask that to generate the thumbnails. If that crashes, it will crash Opus as well, but the bug is in the thumbnailer, not in Opus.
From this other recent thread, it looks like there is a problem with PDF-XChange at the moment:
You could report the issue to the PDF-XChange team, or try a different PDF viewer to avoid the problem.
If you want to report the issue to the PDF-XChange authors, the crash dumps may help them locate the problem in their DLL. You can find them as shown here: Automatic crash logs (for bug reports)