Azure Functions: "Invocation of cloud script function NewFunction failed"

I always get this error message in the client code: "Invocation of cloud script function NewFunction failed".

I get no event in PlayStream when the error happens. What does this error message mean and what is wrong with my code?

How can I find out if my Azure function works correctly? I don't know if there is a problem with my Azure function or with the client code that calls the Azure function.

Client code:

var result = await PlayFabCloudScriptAPI.ExecuteFunctionAsync(new ExecuteFunctionRequest()
{
    Entity = new PlayFab.CloudScriptModels.EntityKey()
    {
        Id = entityid,
        Type = entitytype,
    },
    FunctionName = "NewFunction",
    FunctionParameter = new { NewDisplayname = "Sydney" },
    GeneratePlayStreamEvent = true
});


if (result.Error != null)
{
         
}
else
{
    if (result.Result.FunctionResultTooLarge ?? false)
    {
        Console.WriteLine("This can happen if you exceed the limit that can be returned from an Azure Function, See PlayFab Limits Page for details.");
    }


    Console.WriteLine("The " + result.Result.FunctionName + "function took " + result.Result.ExecutionTimeMilliseconds.ToString() + " to complete.");
    Console.WriteLine(result.Result.FunctionResult.ToString());
}

Azure Function:

namespace My.Functions
{
    public static class NewFunction
    {
        [FunctionName("NewFunction")]
        public static async Task<dynamic> MakeApiCall(
            [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = null)] HttpRequestMessage req, ILogger log)
        {
            /* Create the function execution's context through the request */
            var context = await FunctionContext<dynamic>.Create(req);
            var args = context.FunctionArgument;


            var desireddisplayname = args["NewDisplayname"];
 
            /* Create the request object through the SDK models */
            var request = new UpdateUserTitleDisplayNameRequest();
            request.PlayFabId = context.CallerEntityProfile.Lineage.MasterPlayerAccountId;
            request.DisplayName = desireddisplayname;
            /* Use the ApiSettings and AuthenticationContext provided to the function as context for making API calls. */
            var adminApi = new PlayFabAdminInstanceAPI(context.ApiSettings, context.AuthenticationContext);
         
            /* The PlayFabServerAPI SDK methods provide means of making HTTP request to the PlayFab Main Server without any 
             * extra code needed to issue the HTTP requests. */
            return await adminApi.UpdateUserTitleDisplayNameAsync(request);
        }
    }
}

I have chosen "Authorization level: Anonymous" when I created my Azure project in Visual Studio Code. And I have added "PLAYFAB_DEV_SECRET_KEY": "...", and "PLAYFAB_TITLE_ID": "..." to my local.settings.json file.

In addition, I selected the Register Function button in my PlayFab account and I have created the following cloud script function:

Trigger type: HTTP

Function name: NewFunction

Function URL: https://myplayfabfunctionapp.azurewebsites.net/api/NewFunction