iCal events no longer trigger SelfControl

Hi,

I am using a late 2011 MacBook Pro 13" running OSX 10.8.2.

I used Automator to make a .app for Self Control to block access to particular websites at eight hour intervals. I then created iCal events and used the "open file" alert type in order to trigger the .app at particular times.

This system had been working fine until recently. Now, the system will work for a few days, whereafter iCal will return an error message rather than triggering the .app file. This is the error message that is returned:

http://s8.postimage.org/tceb7xd79/Screen_Shot_2013_02_28_at_14_32_24.png

Upon restarting the laptop, iCal will work correctly again for a few days, before the error message is again shown instead of running the file.

Could anyone shed any light over why this is happening and how I can solve the issue?

Thanks very much.