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/maps1/15337/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-4363b134-71b8-496a-a197-9c69401d0005",
      "cw-3f294778-24b7-48e6-a260-afbaca5756c6"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-7a57a5d7-4ba7-4ae9-8590-eb8654b32bde",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/15337/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-3f294778-24b7-48e6-a260-afbaca5756c6"
  }
}