First of all, are you using an administrator account, when you are entering the serial number? Aperture needs to write the information to the system library, not the user library. Try to log in from an admin account while entering the serial number. If that does not help, you may have a corrupted "ProAppSystemID" file or Aperture cannot write to the System Library.
First check, if you still have an old file "ProAppSystemID" in your System Library.
- Quit Aperture.
- Go to your MacintoshHD and open the folder "Library", then "Application Support", then "ProApps".
- Remove the File "ProAppsSystemID" to the Desktop, if it exist and restart Aperture.
- Reenter your serial number, hopefully for the last time.
Sometimes Aperture writes this file to a temporary directory, when it cannot write to the Library. If the above does not work, look at the temporary directory /tmp
Use the Finder's "Go" menu
Go > Got to folder and enter /tmp into the textfield. If you see the file there, move it to its proper location.
Check the permission on the folder "Library", then "Application Support", then "ProApps". You (as admin) need to have read/write access to all flders along the path.
Regards
Léonie