since about a week I have some Icons in the Dock and in the Launchpad (e.g.: AppStore, Mission Control) with low-res icons.

However if I start the app (in this case App Store), the correct (hi-res) icon is displayed:

After quitting the application, the low-res icon will be displayed again.
I have the same symptoms for a handful of apps (mainly Apple applications).
What's the cause of this, and how can this be fixed to ensure hi-res icons are shown?