A fix for “An error occurred”
*Note Temp files are located at: ~/Library/Application Support/AppStore/
To fix the “An Error has occurred” perform the following:
1. Sign out of the Mac App Store (Store Menu -> Sign Out)
2. Quit the Mac App Store app
3. Delete anything underneath ~/Library/Application Support/AppStore/
4. Restart the Mac App Store app
5. Sign into the Mac App Store (Store Menu -> Sign In)
After signing back in, you should see your downloads start over.
It also would seem there is a debug menu you can do a safer alternative:
1. Open Terminal
2. Enter
defaults write com.apple.appstore ShowDebugMenu -bool true
3. Relaunch the AppStore
4. Choose the menu item Debug Menu -> Reset App
5. Choose the menu item Debug Menu -> Check for Available Downloads…