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/maps1/20681/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-d205cd1d-ab37-4225-a127-d0b0a3f756ff",
      "cw-9b97e9ed-fb42-4283-a177-253c349c7def"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-6352cfe4-f188-4b70-8de6-9fbbccf3c69e",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/20681/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-9b97e9ed-fb42-4283-a177-253c349c7def"
  }
}