Datafile is Already Open for Exclusive Access

  For a given DECwindows Calendar datafile, the access
  granted depends on the file protection of the datafile.
  DECwindows Calendar supports either one user (session)
  with exclusive write access to its datafile or multiple users
  (sessions) with read-only access.

  Mixed read and write access to the same datafile is not
  possible.  You can open any read-only datafile or any write-
  access datafile that is not currently opened by another user.

  In your case, another DECwindows Calendar session has
  already opened the specified datafile for exclusive write
  access, so your request to access it is denied.

  If you must open the datafile, ask the current user to close it.
  Otherwise, try again later.