The serial number shouldnt be registered to anyone else
Indeed, but these are user to user forums, not any contact for Magix staff so no-one here can really definitively help you with this problem, sorry.
However, you need to ensure that the email address in your Magix account is exactly the same as the one you are now using to register the software. If the account address is "wrong" you can use the section User data to update and correct it.
If you cannot access your account you need to email full details to infoservice@magix.net and ask for their help in resolving this.