I realize this is an old thread, but I am having the same problem and I found the following answer in an archived discussion. The problem is, I am not quite sure how to use Terminal and I tried following the instructions but couldn't figure it out. Any help out there?
6 oct. 2009 13:53 (in response to Richard Collyer1)
Got the same problem here.
Seems I got rid of it by updating my LaunchServices database :
1) Open the Terminal, and log in with an admin account (by using su and/or sudo for example) if your current OSX account does not have admin privileges.
2) Go to the LaunchServices.framework' Support directory :
*cd /System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.fra mework/Support*
+(Note: in previous OSX versions, it was a subframework of ApplicationServices and not CoreServices, the path has changed since)+
3) Use the "lsregister" commande line tool found in this directory to do a rescan/rebuild of your LaunchServices database. You can e.g. update your whole database with this command :
*./lsregister -r -apps local,system,user*
+(Or you can specify the path to the ScreenSaverEngine.app itself directly as a parameter)+
After the LaunchService database has been rebuild, you should be good with ScreenSaverEngine (you can use an active corner to launch it immediately and check if the fix worked).