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/maps1/23020/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-f08f4f40-496f-44c6-9f53-d37a05da78f3",
      "cw-46377f92-7ac4-450b-9fde-3c0cc6fdd235"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-b2971dd6-bb85-4d76-be48-91242c36b936",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/23020/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-46377f92-7ac4-450b-9fde-3c0cc6fdd235"
  }
}