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SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI6073462%2Fmanifest",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-2251755d-fa8b-4697-94f3-687f8e22877d",
      "cw-29ea8e0e-81af-4710-82cc-7b923c8d8a41"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-2fbcd027-b7b9-4970-ade2-98428e5eddfa",
    "manifestId": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI6073462%2Fmanifest",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-29ea8e0e-81af-4710-82cc-7b923c8d8a41",
    "loadedManifest": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI6073462%2Fmanifest"
  }
}