I used Task Manager to stop everything marked Dopus, then install worked.
My guess is that either Opus didn't quit when the installer asked it to, or antivirus was blocking the new binaries from replacing the old ones (which is less likely if it worked OK after manually killing Opus).
What happens if you run the installer again over the top of itself? Does the problem happen again or was it a one-off?
I ran the installer again, it worked fine.