If you haven't already, please see the Problems installing Opus guide, in case anything in there is helpful. The exact error message isn't in the guide but some similar ones are and it's possible one of the suggestions will still be what you need, or might provide an idea of what to try.
If none of that helps, using Process Monitor (there's a link to it in the guide) to log what the installer does up to the failure point should provide us with a good idea of what is failing.