Angular: AppwriteException: Database not found

I have a database & collection pair set up with the database ID "Data" and the collection ID "Collection". They are set up with All users granted all access rights to the collection.

Database and collection are created & configured by a backend application (via the .net API). This application is able to create new documents in the collection just fine. However when the Angular-based client attempts to do so, it is met with the exception "AppwriteException: Database not found". This same client can see the documents created by the backend just fine, via a realtime subscription.

The frontend is registered on the (cloud.appwrite.io) dashboard as a web client with hostname set to "localhost" as I am just testing & developing locally for now.

The typescript attempting to create the document in the client is as follows:

  async sendMessage (message: string)
  {
    this.error = null;

    try
    {
      await databases.createDocument (
        'Data',
        'Collection',
        ID.unique (),
        {
          sender: this.username,
          contents: message
        }
      );
    }
    catch (error)
    {
      this.error = error;
    }
  }

Any idea what could be causing the error? Common pitfalls I should look out for?