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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/maps1/12237/manifest.json",
    "isFetching": false
  },
  "window": {
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    "companionAreaOpen": true,
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      "cw-5fff16fb-c69a-4496-9ccd-714f9f0bb1a9"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-46816a94-ffde-4bcc-ae02-227383f9d4b2",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/12237/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-5fff16fb-c69a-4496-9ccd-714f9f0bb1a9"
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}