Shortcuts says action contains invalid Metadata

I am trying to include AppIntents into my app. To achieve this, I am following Apple‘s WWDC Session. Everything works great until I try to add a parameter to my intent.

My Entity and Query look like this:

struct ActivityEntity: AppEntity {
	var id: String
	var name: String

	static var typeDisplayRepresentation: TypeDisplayRepresentation = .init(name: "Activity")

	var displayRepresentation: DisplayRepresentation {
    DisplayRepresentation(title: LocalizedStringResource(stringLiteral: self.name))
	}

	static var defaultQuery: MobilityActivityQuery = MobilityActivityQuery()
}

struct MobilityActivityQuery: EntityQuery {
	func entities(for identifiers: [String]) async throws -> [ActivityEntity] {
    Activity.all()?.compactMap({ activity in
        identifiers.contains(where: { $0 == activity.name }) ? ActivityEntity(id: activity.name, name: activity.name) : nil
    }) ?? []
	}
}

My Intent looks like this:

struct StartSessionIntent: AppIntent {
	static var title: LocalizedStringResource = "Start Recording"

	@Parameter(title: "Activity")
	var activity: ActivityEntity?

	func perform() async throws -> some IntentResult & ProvidesDialog {
    var activityToSelect: ActivityEntity? = self.activity
    if activityToSelect == nil {
        activityToSelect = try await self.$activity.requestDisambiguation(among: Activity.all()?.map { ActivityEntity(id: $0.name, name: $0.name) } ?? [])
    }
    guard let selectedActivity = Activity[activityToSelect?.name ?? ""] else {
        return .result(dialog: "Failed to get activity")
    }
    Session.shared.setActivity(activity: selectedActivity)
    Session.shared.startSession()
    return .result(dialog: "Okay, starting to record this mobility")
	}
}

As soon as the perform method accesses activity, the execution fails and I get the notification from Shortcuts saying: The action contains invalid metadata.

In the console the only message I get is this: [Execution] Unknown NSError Communication with the Helping-App is not possible. (this might be a bad translation from me)

I am completely clueless on how I can debug this problem and my internet search didn‘t help either. Any help is much appreciated. Thanks in advance