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/maps1/23733/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-d78cc83f-b598-4c84-8bf8-62e0232d0949",
      "cw-497fb408-e43f-440f-87d7-a096798a6659"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-bbb66bef-abb0-4777-bf22-ba9d771db5a3",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/23733/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-497fb408-e43f-440f-87d7-a096798a6659"
  }
}