No problem. I try and be pretty prompt when people are having issues.
I just tried it on my end and it worked, but I also did a couple of things I don't normally do. We'll try these things on your end.
First we'll uninstall Nthusim HMD. Run the installer and select the uninstall option. Once completed go to the next step.
Next thing we'll go into the control panel and into "Folder Options".
Open "Folder Options", then select the "View" tab.
Under "Hidden files and folders", set to" Show hidden files, folders, and drives".
That allows us to see some of the hidden folders.
Close out the control panel and open a Windows Explorer file window for your C: drive.
With hidden folders now showing you should now see a "ProgramData" directory. Open that folder.
You should now see an "ABAS Labs" directory. This is the common directory Immersaview uses the house their licenses. Delete the "ABAS Labs" directory. A bit of useless trivia -- ABAS Labs is what Immersaview used to be called before 2007. It's Andy Boud and Alex Streit's initials.
Exit back to your C drive.
Go into the User directory and enter into your account directory. Within that directory you should be able to now see a directory called "AppData" this was also unhidden with the folder option we did earlier. In this directory is often preference files and program caches.
Open the "Local" directory. There should be an "Nthusim" directory there. This is the directory that is used to house Nthusim preference and configuration files. You can go ahead and also delete it too.
What we did was uninstall, remove any license file and any configuration files. This should remove all potential conflicts.
Now go ahead and run the installer again then attempt to activate. Let me know how things go.
My guess is something old from a previous early access build was conflicting with activation. Maybe the license from early access.