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/b28c3rs3cn8j.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-641e6443-3bca-4208-9d7f-d58c58dd7e20",
      "cw-c5b31776-3d13-4b32-a03e-fc06e737f89f"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-4556e90f-5c1f-4f2b-acfb-93fc2402a75b",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b28c3rs3cn8j.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-c5b31776-3d13-4b32-a03e-fc06e737f89f",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b28c3rs3cn8j.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}