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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/maps1/13953/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
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      "cw-fef32c0f-5e21-47a8-b985-8a5e8b678160"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-9e30b28c-7596-421e-986d-9091b85ca0f1",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/13953/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-fef32c0f-5e21-47a8-b985-8a5e8b678160"
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}