The Error 24 message usually occurs due to a typo in the emaill adress being used or the program was initially registered to a different email address to the one you are now trying to register it with.
If you have changed email address then there are 3 options:
register the program to the old address, it does not have to exist anymore. The downside is, if the old address no longer exists, you cannot use the 'Forgotten password' recovery option.
You can change the email address in your acount at magix.com - login there using the old email address and password to do this however . . . .
If you have created a new account to post for the forum using the current email address and do not want to do option 1, or have never registered the software with an older email address, then you need to email Magix at infoservice@magix.net and ask them to merge the 2 accounts and transfer your registered product(s) to your current email address or look at the issue you are having