I have tried to install version 11.15 several times, but show 11.13 at the end, my current version. This happened with Windows 8.1 and now 10.
Check that your anti-virus scanner is not blocking the installation, and that the file permissions on the drive you're installing on are OK.
You might want to backup your configuration (within Opus, Settings > Backup & Restore), then uninstall Opus (note: this will wipe your config, so do the backup first!). Verify that C:\Program Files\GPSoftware has now been deleted, and if not delete it manually. Verify that you can also create a new test folder below C:\Program Files. Then try the installer again.
Make sure the installer came from the official downloads page, and that it shows the correct version number in the titlebar when you are running it. (If it does not, try emptying your internet cache, or using a different web browser to download it).
Thank you for your help. I now have my latest version.