Some users have experienced a Sharing Violation Error when opening the Planner application.  The typical cause  for this error is that there is an instance of Planner already running or the Planner was previously close but still resides as a running process.  There are two methods to fix this issue when it occurs.  The first method  is to restart your computer and to open Planner again.  The second method is to press CTRL-ALT-DELETE and select Task Manager.  Once the Task Manager is open, look for Planner.exe as a running process, right click on it and select END TASK or END  PROCESS.  Open Planner from the shortcut, note the program should take 10-30 seconds to open, do not try clicking on the shortcut again within this initialization period, otherwise you may get the Sharing Violation Error again.