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 http://www.nhoj.co.uk/icalreplychecker

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/iCal.app/Contents/Resources/Scripts

Lesson learnt: take care when trying amature software.