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/maps1/6657/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-61f6a035-20c7-4d63-ad3d-82e3eef5360a",
      "cw-972470ea-3580-430f-93d0-2ad11ef76093"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-36f63303-1781-4fda-bcfe-45aeb0363dc9",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6657/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-972470ea-3580-430f-93d0-2ad11ef76093"
  }
}