I can add a new document to Firebase Firestore but not update one, why?

Question

I'm using Firebase's Firestore database and haven't had any issues until today - I have no problems reading data or adding new documents to the collections. What I can't seem to do now is update an existing document (which was working previously).

What could be causing this?

Code for the update >

    func updateSite(site: Site) -> Bool {
    let db = Firestore.firestore()

    do {
        try db.collection("Sites").document(site.id ?? "Unknown ID").setData(from: site)
        return true
    } catch {
        print(error)
        return false
    }

}

Edit- I'm working in Swift (iOS) via Xcode 13.3. There haven't been any changes to the code to deal with Firestore but I did switch to using Firebase-cloud-storage for images, other than that nothing I can see impacting here.

The error on update is returned as an Auth Error - (See below)

: 2022-03-23 06:06:44.462999+1300 Scout[15793:933890] 8.13.0 - [Firebase/Firestore][I-FST000001] AppCheck failed: 'The operation couldn’t be completed. Too many attempts. Underlying error: The operation couldn’t be completed. The server responded with an error:

• URL: https://firebaseappcheck.googleapis.com/v1beta/projects/(edited) 9:exchangeDeviceCheckToken
• HTTP status code: 400
• Response body: { "error": { "code": 400, "message": "App not registered: 1:587919427052:ios:ad2ac8f3abd4cbd5fc6cc9.", "status": "FAILED_PRECONDITION" }

As far as I can see there aren't any denied requests showing in the Firebase web console and no other Firestore requests (reads / Writes) from the App are preformed without issue.