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SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

{
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    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https://digital.libraries.psu.edu/iiif/info/maps1/20470/manifest.json",
    "isFetching": false
  },
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    "highlightAllAnnotations": false,
    "id": "window-40d4a7c0-b2e6-400b-9ff0-04a85215b2ed",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/20470/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-3290d70f-96c2-47ec-9b6c-b18ccb94effc"
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