Work out which plugin or extension is causing the crash:
Viewer plugins and shell extensions are usually involved with crashes (etc.) when viewing particular directories. These may come with Opus but most often come as part of other software.
These days, shell extensions are more likely to cause problems than viewer plugins, but viewer plugins are much quicker and easier to test so it still makes sense to rule them out first.
Go to Preferences / Viewer / Viewer Plugins and make a note of the plugins which are currently enabled (so you can restore them later).
A good first bet is to try disabling the Movie plugin. If you find this cures your problem then the issue is likely due to a bad video codec installed on your system. (You should be very careful when installing video codecs, or codec packs, as they can clash with each other and some versions of video codecs are very buggy.)
You may also find the separate Problems with AVI Files FAQ useful.
If you’re still having problems then try disabling all of the other plugins. If this fixes it then you should try re-enabling them until you’ve worked out which one is at fault.
Shell extensions are a type of plugin that work with both Explorer and Opus. When they go wrong they can crash either program, although they may only crash one or the other due to slight differences between the two (as well as the fact that while all shell extension authors test using Explorer, only some perform additional tests using Opus).
You can use a tool called ShellExView to see which extension are on your computer and try disabling some of them. (Disabling extensions via ShellExView is supported by Opus 10 and above, but not earlier versions.)
A good first step is to try disabling (or uninstalling, if they are unwanted) any non-Microsoft extensions. This is especially true for any extensions which may interact with the network or with video/audio files, e.g. ones which talk to source-control servers or “cloud” storage services. The extensions mentioned in the Known Issues list above are also worth disabling at the start of your search, in case they are causing you problems as they have many others.
After disabling extensions, it is best to reboot (or at least exit Opus, but don’t just close all the Opus windows – it continues to run in the background by default – you need to fully exit Opus) in case they had already been loaded before they were disabled.
Some Microsoft shell extensions may also be worth disabling (and occasionally cause problems), especially ones which are part of products like Office rather than part of Windows itself. But even ones that are part of Windows have caused problems in the past, especially with Windows 10.
If the problems go away with most of the extensions disabled, you can then try enabling some of them again to see if you can narrow down the problem to a particular extension. It’s sometimes quicker to do this in batches, e.g. re-enable half the extensions and see if the problem comes back; if it does, you know the problem extension is in that half; if it doesn’t, it’s probably in the other half.
If you do find a problematic component, see if there’s an update or hotfix for the piece of software it’s a part of. It’s often (though obviously not always) the case that such problems have already been fixed by the software vendor and you just have to work out which thing needs updating.