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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%2FMPI140909%2Fmanifest",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-37212acf-edbb-442a-8a32-b4f3d7956207",
      "cw-506a06bd-a26b-425c-b01a-d02deb5327a8"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-9893250e-ad28-4cbb-9787-9eba8b5b6a6d",
    "manifestId": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI140909%2Fmanifest",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-506a06bd-a26b-425c-b01a-d02deb5327a8",
    "loadedManifest": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FMPI140909%2Fmanifest"
  }
}