Mirador viewer

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-98f66659-b3e4-4cf5-ba0b-9d5d5ec4d302",
      "cw-7bf58395-b906-4fd8-92c7-76ced981a79a"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-3e101666-6539-4753-bae1-97587b1d7a13",
    "manifestId": "https://www.rammses.it/wp-content/manifestoso.json?type=sbn",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": true,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-7bf58395-b906-4fd8-92c7-76ced981a79a",
    "loadedManifest": "https://www.rammses.it/wp-content/manifestoso.json?type=sbn",
    "canvasIndex": 15
  }
}