El Captain update download doesn't continue after restarting the Mac

In App Store, selecting Store > Check for Unfinished Downloads did work for me.

I've found the solution:

1. Close the "App Store" App
2. Open "Terminal"
3. Enter defaults write com.apple.appstore ShowDebugMenu -bool true
4. Open the "App Store" app
5. Now you have in the menu-item "Debug" - open it - and open "Show Download Folder"
6. Delete all files and folders in the folder "com.apple.appstore"
7. Close the "App Store" app
8. Restart the Mac
9. Clean the trash bin
10. Open the "App Store" app an repeat the download.

Now the download should work!

If you want to hide the debug-menu again just enter in Terminal:
defaults write com.apple.appstore ShowDebugMenu -bool false