I registered the program in June and have used it since then and now it want me to register again but I get error code 24 which says "the serial number has already been registered to another email address. Please contact MAGIX support.
Welcome to the Magix user to user forums, so not a contact for any Magix staff and no-one here can really help you, sorry.
Apart from suggesting you log into your Magix account, go to My products, find the listing for whichever version of ACL you have and deactivate any installation you no longer use. Then in ACL, making sure you use the correct serial number and email address as you used when you originally registered, try again to register your software.
If that fails, please email infoservice@magix.net, explain the problem and ask for their help.
I followed your suggestion but used the same email address as I use when login in here and it worked. The original email adress did not work or some reason?