Slim in PHP 8.1 shows strange errors

I have been using Slim PHP framework to provide my API service (https://api.rsywx.com).

Recently, I updated my PHP to 8.1 and when I visited the site, it gives the following error:

{
    "statusCode": 500,
    "error": {
        "type": "SERVER_ERROR",
        "description": "ERROR: Return type of App\\Application\\Actions\\ActionPayload::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice on line 67 in file \/home\/tr\/www\/api.rsywx.com\/src\/Application\/Actions\/ActionPayload.php."
    }
}

I have not changed anything in my source code, so I guess the problem raises from Slim framework itself.

Any hints to solve this issue?

Can you please report this bug here? Issues · slimphp/Slim · GitHub