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/maps1/21303/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-60fb40fd-2128-4ec6-88d9-0bf455d79a01",
      "cw-df678410-1555-455f-ac65-134f1a8537b8"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-cb3df54c-2373-4d51-aea8-da62fdcb17f3",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/21303/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-df678410-1555-455f-ac65-134f1a8537b8"
  }
}