Qt5 .desktop files for apps accept qwindowtitle, not caption.
However, the 17.04 branch of ksudoku is still kdelibs4 based, and the
patch applied to KDE BUG 381087 broke the launch of ksudoku here:

--- a/src/gui/org.kde.ksudoku.desktop	2017-06-13 02:44:11.000000000 +0200
+++ b/src/gui/org.kde.ksudoku.desktop	2017-07-15 11:58:32.027477614 +0200
@@ -50,7 +50,7 @@
 Name[x-test]=xxKSudokuxx
 Name[zh_CN]=KSudoku
 Name[zh_TW]=KSudoku
-Exec=ksudoku %i -qwindowtitle %c
+Exec=ksudoku %i -caption %c
 Icon=ksudoku
 Type=Application
 X-DocPath=ksudoku/index.html