This turns out to be a problem with the Constant Contact plugin after all, so I’m moving my complaints there.
We’re having the same issue. Once this was deactivated, magically we could add links in the visual editor again.
This is the console error I’m getting after I click the link (doesn’t throw it in Firefox, but doesn’t work in Firefox either):
Uncaught SecurityError: Blocked a frame with origin “https://public-api.wordpress.com” from accessing a frame with origin “http://www.mysite.com”. The frame requesting access has a protocol of “https”, the frame being accessed has a protocol of “http”. Protocols must match.
