Do I have to get a new serial NR ?
No, you do not need a new licence.
You may have a corrupted "ProAppSystemID" file.
To stop Aperture asking for the serial number, 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.
You may have a permissions problem with your system drive. Open Disk Utility and check the permissions; if Disk Utility reports a problem use the "Repair" button to repair the permissions.
Then try again.
Perhaps the Caches contain a corrupted file. Try to remove
from your user library.
Do you now have a copy of ProAppSystemID in your library? If not, have a look at syncopator's post in this thread: