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/b2vg2mb1n33d.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-bf9f9889-c054-4edb-8f8c-dcad7940e796",
      "cw-bdd08945-e1a6-4463-8c97-3057e2af238e"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-529c9c0b-3e81-4f26-87dd-40d4e7b9a56f",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2vg2mb1n33d.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-bdd08945-e1a6-4463-8c97-3057e2af238e",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2vg2mb1n33d.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}