Screens by Edovia has a rather annoying problem in OS X Lion 10.7.4 and 10.8 (to be fair, it’s Apple’s fault here). Basically a system update (Apple Remote Desktop Update 3.6) causes this problem, and downgrading to ARD 3.5.3 will fix it.

There is a workaround specified in this thread which specifies this:

  1. In “System Preferences -> Sharing”, turn off “Screen Sharing”
  2. Enable “Remote Management”
  3. In the “Remote Management -> Options…”, check the checkboxes “Observe” then “Control”