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 &amp; Privacy -> Accessibility ) and this issue will go away.  (hat tip to @elmigranto for this addenda)