Option --caption makes kdialog crash, or simply not launching because it's considered as not an option. This WAS an option on kde4. If the option is not really needed, it has to be just ignored, and not makes crash or not launch. Olds scripts containing this will now just not work because of that.
The crash is fixed in Qt 5.8.0. The option parser in Qt does not ignore unknown options, but prints an error message: Unknown option 'caption'. This behaviour is consistent with other applications, especially based on GNU's command line option parser. E.g. try "ls --test", no files are listed, only an error message is shown. Please port the scripts to --title *** This bug has been marked as a duplicate of bug 375936 ***
Ok. Now, it's quite a shame that Kdialog label can't be removed from title bar. Sometimes dialogs are little, and having the kdialog 's label is not really welcome. I sat that because earlier, putting the same argument to --title and --caption gives an unique title.
The application name is always added by the Qt platform code. http://doc-snapshots.qt.io/qt5-dev/qwidget.html#windowTitle-prop