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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/maps1/6504/manifest.json",
    "isFetching": false
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    "highlightAllAnnotations": false,
    "id": "window-bf830bab-cbe1-4118-a725-2b5b79d4d00b",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6504/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
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