Error when trying to display a custom block in the widget area

I created a custom block and added it to a widget since you now can use Gutenberg in the widget area with wordpress 5.8.

I can add the block, and even edit it, and it is correctly shown on the front-end. But in wordpress in the widget area it will not render the preview, it shows the error:

Error loading block: Invalid parameter(s): attributes

This is the complete error that comes back from the API request:

{
    "code": "rest_invalid_param",
    "message": "Invalid parameter(s): attributes",
    "data": {
        "status": 400,
        "params": {
            "attributes": "__internalWidgetId is not a valid property of Object."
        },
        "details": {
            "attributes": {
                "code": "rest_additional_properties_forbidden",
                "message": "__internalWidgetId is not a valid property of Object.",
                "data": null
            }
        }
    }
}