After several tries, I'm stuck with this:
The script (as it's part of the files I want to backup, and locked) was preventing the completion of copying.
So I decided to move it in FastScripts menu and run it from there.
Now the script shows twice in System Preferences and I can't find any way to remove the old one.
I had deleted all copies and aliases of the applet and restarted the Mac dozens of times, but no matters.
The fact that it is listed 2 times seems to prevent the normal behavior of the notification:
The buttons are displayed but don't work and the script restarts continuously.
What I'm doing wrong?
And how to get rid of those duplicates in System Prefs?