createOrReplaceInventoryItem replies "Invalid value for ."

I'm trying to call the createOrReplaceInventoryItem endpoint of the Ebay APIs, but the API response is a 400 error with the description "Invalid value for .", so no fieldName is specified, only a dot. It worked since friday, I've not changed anything in thi API call. I've tried the API explorer with the Basic example proposed literally by Ebay: first of all, the "aspects" region is wrongly written, but also if you remove it (or write it correctly), also the Api Explorer reply with a 400 error "Invalid value for .".

As I said I used the ebay's example into the ebay's tool, I post anyway my own payload, with which I receive the same error.

{
    "availability":{
        "shipToLocationAvailability":{
            "quantity":3
        }
    },
    "condition":"NEW",
    "product":{
        "description":"SRED - gr 22",
        "imageUrls":[
            "https://www.wywebsite.it/Images/products/"000.jpg"
        ],
        "title":"Moresilda Red gr 22",
        "ean":[
            "1234567890"
        ],
        "aspects":{
            "Variante":[
                "SRED - gr 22"
            ],
            "Brand":[
                "Blue Fox"
            ]   
        }
    }
}