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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/maps1/19011/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
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      "cw-b798688a-a5e2-46ea-a378-c2b06e3e68d1"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-1f1d90ca-6787-4963-88b7-a83e9e3fad6b",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/19011/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-b798688a-a5e2-46ea-a378-c2b06e3e68d1"
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}