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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%2FE041885%2Fmanifest",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-43a50d4c-6b88-437e-9fd1-c8d9b0cf4eb8",
      "cw-4d691d6a-56fd-45c8-a58b-e54227f6e794"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-70a4eeb7-2c06-4275-a460-f62aa08b87cf",
    "manifestId": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FE041885%2Fmanifest",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-4d691d6a-56fd-45c8-a58b-e54227f6e794",
    "loadedManifest": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2FE041885%2Fmanifest"
  }
}