Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
The Best Free Screensaver For Mac's
You may remember a few years back at WWDC when Apple showed of the app wall which updated with all the logos of the the top apps in the Apps store and there is now a screen saver in exactly the same style.
You can download the screensaver for free from here and requires no more than two clicks to install and get setup. You have the choice if you want the screensaver to display top paid apps or top free apps.
The screensaver itself simulates icons from iPhone apps that have been seen in past editions of apple’s world wide developer conference and the apps on the screensaver slowly change giving you an impressive shimmering effect.
You can currently changed between paid and the free version and this screensaver will defiantly be popular with those who like icons and who are fans with WWDC events.