Apple - Keychain won't let copy passwords after 10.11.1 update

Third-party utilities such as Alfred, TextExpander, or MagicPrefs might appear to take "control" of the window as far as the OS is concerned. You can find the culprit by disabling them all, and turning them on one by one until you find one (or more) which affects Keychain in that manner.

You can add the offending program to the list of approved apps ( System Preferences -> Security & Privacy -> Accessibility ) and this issue will go away. (hat tip to @elmigranto for this addenda)

I had this problem trying to edit the keychain while screen sharing. Even when I had direct access to the server, if the other computer was still screen sharing, it would fail in this way. After stopping the screen sharing and making the changes directly on the server, it worked.