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/b26z1nt4b47b.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-8af36e11-6f2a-4abb-adec-7665a9e9dcdf",
      "cw-ec49c384-83ec-45f0-8dce-141fb17281a2"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-52d5459f-a85c-4a7d-874b-880fc51b8d0c",
    "manifestId": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b26z1nt4b47b.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-ec49c384-83ec-45f0-8dce-141fb17281a2",
    "loadedManifest": "https://iiif-collection.lib.uchicago.edu/object/ark:61001/b26z1nt4b47b.json",
    "canvasIndex": 2,
    "allowMaximize": false
  }
}