Is it possible that you already used the source “myApp” when writing to the standard Application log? If so according to MSDN:
If a source has already been mapped to
a log and you remap it to a new log,
you must restart the computer for the
changes to take effect.
http://msdn.microsoft.com/en-us/library/2awhba7a.aspx
(about half way down the page)