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

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2kc4bv6dq74.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-fcd06afb-740b-4342-95cb-4f2c3f0e9ac2",
      "cw-02e275af-ad5f-43b5-897a-67f357f86b9d"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-4e15d266-de06-4cc9-9d0e-fd21173bef51",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2kc4bv6dq74.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-02e275af-ad5f-43b5-897a-67f357f86b9d",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2kc4bv6dq74.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}