One more question: how do I know, which phonom-backend I use in krusader?
go to system-settings (Systemeinstellungen) -> multimedia -> Backend,
The first, the top one is the backend which is in use.
And than please take a look to Systhemeinstellungen--> Anwendungs- und Systembenachrichtigungen --> Wiedergabe-Einstellungen --> keine Audioausgabe <-- please do that. Deactivate all system-message-sound-signals.
And test if knotify crashes when you are deleting a file via krusader.
btw. I think you are using the gestreamer-backend, Maybe you can try the vlc-backend also and see if KNotify crashes when you are deleting a file via Krusader.