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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/maps1/20916/manifest.json",
    "isFetching": false
  },
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    "highlightAllAnnotations": false,
    "id": "window-02decb96-64dc-4ada-b3fd-befd1e7a626c",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/20916/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-523a088e-39e8-411f-9f4c-009ac75c3d3c"
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