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

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https://www.rammses.it/wp-content/manifestoso.json?type=sbn",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-db54c88d-d161-42ba-8306-c89baeb29a5d",
      "cw-e271b926-0fb8-48a3-a37a-3cbbd3400010"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-40d79cfe-a5a9-462f-8184-88e74c4f8313",
    "manifestId": "https://www.rammses.it/wp-content/manifestoso.json?type=sbn",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": true,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-e271b926-0fb8-48a3-a37a-3cbbd3400010",
    "loadedManifest": "https://www.rammses.it/wp-content/manifestoso.json?type=sbn",
    "canvasIndex": 15
  }
}