sharepoint online – Remote event Reciever app is showing this error “[SecurityTokenSignatureKeyNotFoundException: IDX10501: Signature validation failed”

I have a remote event receiver deployed inside Azure, and it is working well for 2 years. but yesterday i noted that the RER is not longer working !! now i went to site content >> click on the App , where i got this error instead of getting the web title:-

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here is the code for the default.aspx page:-

protected void Page_Load(object sender, EventArgs e)
        {
            // The following code gets the client context and Title property by using TokenHelper.
            // To access other properties, the app may need to request permissions on the host web.
            var spContext = SharePointContextProvider.Current.GetSharePointContext(Context);

            using (var clientContext = spContext.CreateUserClientContextForSPHost())
            {
                clientContext.Load(clientContext.Web, web => web.Title);
                clientContext.ExecuteQuery();
                Response.Write(clientContext.Web.Title);
            }
        }