I would check your video codecs since they are almost certainly involved. Something else may have updated them. Nothing related to how Opus generates video thumbnails has changed in a long time, and most of it involves asking Windows and the video codecs to do the main work.
You may find Explorer has problems in the same folder, although it could also be using a different route to generate the thumbnails which may mean different codecs are being used, or in a different way which does not cause the problem.
If you configure the Movie plugin (Preferences / Viewer / Viewer Plugins) there is an option to tell it not to generate thumbnails for video files, which will result in Opus calling the Windows shell for the thumbnails instead. Video codecs are ultimately used in both cases, but one may work better than the other sometimes.
If you have crash dumps corresponding to the date & time of the thumbnail-related crashes, we may be able to use them to tell you which video codec or other component is causing the problem. You can zip & email them to leo@gpsoft.com.au if you wish.