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/maps1/8523/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-3eefd502-2431-47af-9ff4-f8190cfba07d",
      "cw-ee1474a5-3f16-4ffe-9298-25ed9977fda4"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-a0228ee8-7e22-4985-84cc-53a97cf8aa9e",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/8523/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-ee1474a5-3f16-4ffe-9298-25ed9977fda4"
  }
}