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/maps1/5794/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-2a7a530a-987a-4e2c-80bf-f8c6e9501d15",
      "cw-cd182a45-039e-4805-8d73-7fce65993954"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-f9131a5d-5a82-42a4-9288-ad87847231ec",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/5794/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-cd182a45-039e-4805-8d73-7fce65993954"
  }
}