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/maps1/21331/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-6d86de67-5fe1-4080-85f7-e8d06e1eb952",
      "cw-3f31ed5b-5741-4d98-bd71-8080de4fcfee"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-568a9b38-473d-4178-b042-0c7d38c26ccd",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/21331/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-3f31ed5b-5741-4d98-bd71-8080de4fcfee"
  }
}