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Sourcemap: yarn start do not resolve the url

Hello I’m using sage on Windows7 and I have problem with css sourcemap.
In the console I see:
Errore mapping di origine: Error: Invalid URL: .
URL risorsa: blob:http://localhost:3000/2c7cf7e3-6781-46b3-aac7-0cc9afabedca
URL mapping di origine: data:application/json;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbIlM6L3d3dy9hcHBlbm5pbm9tb2RlbmVzZS93cC1jb250ZW50L3RoZW1lcy9hcHBlbm5pbm9tb2RlbmVzZS9yZXNvdXJjZXMvYXNzZXRzL3N0eWxlcy9TOi93d3cvYXBwZW5uaW5vbW9kZW5lc2Uvd3A…

Why? Who could help me?
Thank you
alex