I would heartily agree with you: it sounds like a registry error. I ran a registry cleaner this morning (don’t know how good it is: Advanced Uninstall Pro 13)
Were you able to sort this out?
Hi Steve-thanks for all your help. I’m sorry it took me so long to reply: I literally just saw your last inquiry.
So I had to completely rebuild my system: Reset PC. I’m still replacing apps on it. But that did fix the issue.
A very time/effort expensive way to get some software to work, but that’s just the way it goes.
Hello, I have actually been struggling with this issue for a while now too. It started about a year ago when I tried to reinstall my Cubase.
Now with updating to Cubase 12 I am running into the same issue again. How I have previously been able to bypass it, has been to go to the .exe file and extract its content, running the cubase12.msi file instead of the installer provided by cubase.
However, this time that didn’t work, and the msi installer would give me an error telling me I already had the product installed, or a newer version. Uninstalling via the .msi right mouse menu told me I couldn’t uninstall because I did not have it installed. Right. That is strange.
Running countless of reg fix software and updating windows, and all the stuff mentioned above, never worked.
After some troubleshooting I tried opening CMD as admin, and fired the following:
Msiexec /a "C:\Users\<MyPcUserName>\Downloads\Steinberg\WIN64\Cubase Pro 12.0.20 - Application (required)\Cubase 12\Additional Content\Installer\cubase12.msi" /l*v "C:\msi_log.txt"
This ran the installer! Now, having selected “C:\Program Files” as ‘Network Drive’, it installed Cubase 12 into the right place. Or at least, I hope. As it has also put the AppData in the program files now. Guess I’ll be moving that one manually 
If any Steinberg developer would like to see the log file in order to dive deeper into it let me know.
My guess is that it is related to starting the MSI with admin privileges, via a CMD with also admin privileges. Perhaps my c drive’s temp directory permission is the one causing it, requiring a full admin executed installer in order to use it correctly? Oh well. That is just a wild guess. I hope this helps people into the right direction, whoever is battling the same issue and too far down the rabbit hole to reinstall their windows 