Problem setting Notepad++ as log message editor for svn 1.9.7 (Windows7)

I have configured Notepad++ to be the editor for the log message.
This works OK if Notepad++ is *not* running at the time of an svn
operation. Then Npp pops up an edit window and I can enter the message
as expected and then close the window.

But when Npp is already running with one or several tabs with other
file edits, then Npp pops up with an edit tab inside the already
running window.
Now svn will not recognize my log message but instead pop up the
following on the command line:

Log message unchanged or not specified
(a)bort, (c)ontinue, (e)dit:

If I choose c then the commit proceeds but there is no log message...

How can I use Notepad++ as the log editor and not have this happening?

-- 
Bo Berglund
Developer in Sweden