Nevermind.
There was no problem with the Listview or anything, Visual Studio just showed the errors there because the ListView was inside a custom SCompoundWidget (my hud) and that was inside a APlayerController, which holds the source for the Listview Items (FInventoryEntry), BUT only had a forward declaration of the struct. With UseUnity = true it got the required definition.
Just including the struct header in the my playercontroller header solved everything.