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/maps1/6374/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-c68efb91-c2d9-44f4-9ef5-62c50426b28c",
      "cw-8495bd49-d595-49aa-836d-1287a516dd11"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-cf1f8fa2-8c35-419d-8acd-66d42890821c",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6374/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-8495bd49-d595-49aa-836d-1287a516dd11"
  }
}