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/maps1/3820/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-8bb265b3-7822-41e8-8c30-6137b2319906",
      "cw-1af6eaad-1f3f-4374-95e1-fb9532b3d0f4"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-ec192ec2-cc76-485b-9edb-e38214582c66",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/3820/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-1af6eaad-1f3f-4374-95e1-fb9532b3d0f4"
  }
}