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/maps1/8721/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-5626a5cb-8859-49ee-8285-0f0fbc01a7af",
      "cw-31830783-0b81-45e5-9500-4273bf8f81a2"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-10aeefc1-66b3-4d2f-8485-fed8941cefc7",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/8721/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-31830783-0b81-45e5-9500-4273bf8f81a2"
  }
}