welcome to the Magix Sound Forge (SF) users forum @walter-f.
When you enter your serial number, are you using the same email address that was used when SF was initially registers?
A few things you can try.. based on SF Pro, Audio Studio may vary some.
There is a registry tweak, but many folks wisely do not want to go there, otherwise.. - "Help> Deactivate the Software on this PC". Exit SF, reboot the PC, launch SF and re-activate. - "File Menu> Reset all preferences and reset cache". - Log into Magix Service Center> go to 'My Products' and download the latest SF-12 build. Reinstall SF w/o uninstalling - Uninstall SF with a third-party uninstaller (Revo Uninstall for instance) then re-install.