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/b29b1tv7xn73.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-1052756e-095d-4cae-8e08-629dd6e1463d",
      "cw-e9b47f79-f6c0-48cc-a7f2-db35fc10ea8f"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-16b579ff-2f10-4fb8-be21-e9b52da44614",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b29b1tv7xn73.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-e9b47f79-f6c0-48cc-a7f2-db35fc10ea8f",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b29b1tv7xn73.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}