I've got that error message before, it means the script either was deleted from the database or got corrupted, you may recover it if you rez the same object in another sim, but if you do, copy the script, make a new one and paste the script in the new, empty script, then replace the problem script with the fresh copy.
otherwise you could "fish" the script form another object because when that message appears the original object uses to be lost except in the sims that have it cached.
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