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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%2FC004294%2Fmanifest",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-4be6235b-e80b-438e-a51f-b58f6892e660",
      "cw-2dd3591a-5234-4e78-9ad4-0e1bfb5bf5ed"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-5cd06a8e-bd48-4c71-8217-c6ef3bb0ba26",
    "manifestId": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FC004294%2Fmanifest",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-2dd3591a-5234-4e78-9ad4-0e1bfb5bf5ed",
    "loadedManifest": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FC004294%2Fmanifest"
  }
}