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SyntaxError: Unexpected token 'E', "Error from"... is not valid JSON

{
  "manifest": {
    "error": "SyntaxError: Unexpected token 'E', \"Error from\"... is not valid JSON",
    "id": "https://iiif.participatory-archives.ch/SGV_10P_05232/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-2ce730f2-aae2-4616-b855-41fd5443d61b",
      "cw-d746256b-b0db-4075-8f94-26c4fb61a1e1"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-89b5d288-5b2c-4133-bab4-46ff67f7aa6d",
    "manifestId": "https://iiif.participatory-archives.ch/SGV_10P_05232/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": true,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-d746256b-b0db-4075-8f94-26c4fb61a1e1",
    "loadedManifest": "https://iiif.participatory-archives.ch/SGV_10P_05232/manifest.json"
  }
}