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/b2356md6v073.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-b47fc73d-923b-47f4-a49f-e4be33b9a6c4",
      "cw-17bc4f2c-16d5-4bd1-8d67-824418bb98d5"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-dcba2ac9-d361-4b69-9fb0-5994bcbd3cfe",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2356md6v073.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-17bc4f2c-16d5-4bd1-8d67-824418bb98d5",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2356md6v073.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}