The blue icon should only show if Opus isn’t pinned, unless taskbar grouping is turned off (looks like it is on, though).
A blue icon for an individual window is normal if that window is a dual display. (You can change the icon colors for different states under Preferences / Display / Colors and Fonts: File display icon.)
But when the icon is pinned, it should use the icon for the whole program rather than the icon for any individual window. The shell seems to be confused about what that icon is on your system.
Since you’ve already tried clearing the icon cache and unpin/repin, the most likely thing is that the registry settings which define the App ID are incorrect. Reinstalling Opus over the top of itself should repair the problem if that is the case. Be sure to reboot at the end when prompted.
If that still does not fix it, it might be worth trying an uninstall, reboot, then install. But before using the uninstaller, be sure to backup your configuration (Settings > Backup & Restore) as uninstalling will wipe the config. You’ll have the factory settings afterwards, but can then restore the backup via the same menu.