Mirador viewer

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_10D_00621/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-2a17709d-b7f0-40e5-94de-8f1d7bb8f4ae",
      "cw-3d93a6e9-e522-41da-8574-ccdf98d4209e"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-1fcd037d-3337-48ac-91ec-4e45ba62de42",
    "manifestId": "https://iiif.participatory-archives.ch/SGV_10D_00621/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": true,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-3d93a6e9-e522-41da-8574-ccdf98d4209e",
    "loadedManifest": "https://iiif.participatory-archives.ch/SGV_10D_00621/manifest.json"
  }
}