My iCal crashes everytime when someone sends me an invitation and I try either to accapt or decline it. After some investigation I found out it is the problem with iCal Reply Checker” at

After unstalled this app, it resets the Mail.scpt file to an empty file. To solve the problem: get another copy of Mail.scpt from another Mac, preferably from the same OS X version; and put it inside the folder: /Applications/

Lesson learnt: take care when trying amature software.