Nevermind, the error was on my part. I got the same problems when testing the action myself, even though it weirdly didn't cause any trouble when creating the action.
The problems mainly occured because I wrote a custom inspector for that action, which showed all available Schemas and Item Names, but because there were several problems with writing those values to the action and because it caused lag spikes when loading all data, I removed that custom inspector and re-worked the action, which means you have to
delete the "GDECreateItemInspector.cs" file and
replace the "GDECreateItem.cs" with the one from the attachments or the initial post.
Sorry for that, but it should work now and I tested it more thoroughly this time