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/b2mt6k84q34z.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-1aab735c-63de-408e-8f96-d188f9bcaab8",
      "cw-32ef48b2-872e-4079-866e-0836000d84a7"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-ad0097e6-4785-4ae0-83e4-6343604dd50d",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2mt6k84q34z.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-32ef48b2-872e-4079-866e-0836000d84a7",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b2mt6k84q34z.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}