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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-5aea8831-3e3d-4404-a692-7c961254d309",
      "cw-9f7c9d5a-8758-4166-a261-a7f5a736e73a"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-4a39cc19-1346-4f30-9b28-f5e87bb17009",
    "manifestId": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI6073462%2Fmanifest",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-9f7c9d5a-8758-4166-a261-a7f5a736e73a",
    "loadedManifest": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI6073462%2Fmanifest"
  }
}