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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/maps1/6526/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
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      "cw-734891b7-e68e-44ac-ab4f-0ec407f8f1a9"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-f6ae8ef3-ba1b-4f73-a41c-3d92ec02c06a",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6526/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-734891b7-e68e-44ac-ab4f-0ec407f8f1a9"
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}