I'm trying to install ACID Music Studio 9.0.40 on a new computer with Windows 10 and the installer is telling me the registration failed, so I tried registering with the serial number from "My Products" and it says "Serial number already used".

So I'm stuck between [The installer is telling me to register] and [The website is telling me I don't need to register].

Go to Your account in Magix.com. Click on products and click on "Show more details" under the serial number thats already there. You will see an option to "Deactivate" when you scroll down a bit. It will show you the PC ID that the serial number is attached to. Hit deactivate and then go back to Acid and re-register. This should work.