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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/maps1/27375/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
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      "cw-7c17d004-8f7e-4eb3-b73f-fcae430e6cb3"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-1f34c330-da72-4ebf-a501-3d80b763409a",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/27375/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-7c17d004-8f7e-4eb3-b73f-fcae430e6cb3"
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}