Unable to add folders to Dock Strangely, all folders (only Downloads in my case) have vanished form my dock. A reboot did not help.
I can still drag apps onto it, but not directories. 
How can I find out what the problem is and maybe fix it? Is there some configuration .plist to which I could manually add stuff?

The system is 10.12.3 on an MBP 9,2.
I restored ~/Library/Preferences/com.apple.dock.plist and ~/Library/Preferences/com.apple.dock.extra.plist from Time Machine and killalled Dock and now it works. I suspect the first file is the only relevant one since it contains an entry for the directory.