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/maps1/2521/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-cd730157-aaee-4057-8fc1-4d6fc9ce22e1",
      "cw-14fbdafd-c327-4b7f-bd83-0a8d39e29234"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-e8ca2c1e-4d95-497e-ac6a-eaf9b3bcef98",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/2521/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-14fbdafd-c327-4b7f-bd83-0a8d39e29234"
  }
}