Mirador viewer

SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https://digital.libraries.psu.edu/iiif/info/maps1/3509/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-e3bef130-c389-4c3e-bd38-013f83bf2739",
      "cw-ed119a62-2813-49a3-94c3-edcad008a380"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-88d0e0e2-9013-4714-95c0-f137161b81a9",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/3509/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-ed119a62-2813-49a3-94c3-edcad008a380"
  }
}