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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/maps1/6678/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-1b4f9e97-d974-40f9-bfa5-d067051fd68c",
      "cw-668cd082-ab81-4788-b524-ad0bc0a5735d"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-58236f22-c14f-469c-b737-9eed56d5227b",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6678/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-668cd082-ab81-4788-b524-ad0bc0a5735d"
  }
}