I have another email address that I used to log in with Magix years ago. With my online purchase, I received the serial number associated with the old address.
When I use “forgot my password”, I am informed that the used forgotten-password is invalid. I have been able to obtain a user login using a different email address, but, the registration and activation functions don’t recognize it.
Hmmm. You may have a problem because you have a new account with the email address that you want to use.
The way to do it properly was to simply log in to your Magix account using the old email address (works even if the email address no longer exists - it is just a user name), User Data, Change Details and change the email address. You can still try this. I don't know how it will react to you having a second account.
So, you registered with an old email address that no longer works, have forgotten the password, cannot change the password because to do so, Magix (or any other company) would have to send you an email to the old email address which doesn't work. I would say that your problems are probably much greater than with just Magix. I expect that you have learned something from this. Sorry, I have no ideas as to how to help.