Hi Birthe,
thanks for the quick reply! The plugin's tiddlers are all present (e.g. the one that you mentioned) but even after moving to the trashbin with the trashbin icon (I do have a localized UI, so there are two icons on an open tiddler, "Move this tiddler to trashbin & "Lösche diesen Tiddler" i.e. "Delete this tiddler").
In Kookma's example/tutorial, the deleted tiddlers all have the name of e.g.
$:/trashbin/testJSON - these are the ones that I don't find even after moving tiddlers to the trashbin.
Ta,
Helge.