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_12P_04853/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-ded15fe7-cd9d-4cab-82c7-41fd4556ac21",
      "cw-75c90ef4-5bcd-45c1-b7f5-33e32b30735f"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-3fca5756-e682-404f-b972-e4b5f29b1ebe",
    "manifestId": "https://iiif.participatory-archives.ch/SGV_12P_04853/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": true,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-75c90ef4-5bcd-45c1-b7f5-33e32b30735f",
    "loadedManifest": "https://iiif.participatory-archives.ch/SGV_12P_04853/manifest.json"
  }
}