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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/maps1/19407/manifest.json",
    "isFetching": false
  },
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    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-5158153b-a797-4557-bb66-ed8e9e5035f5",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/19407/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-d25b2eb8-c78b-40ea-ac06-f275f463d60b"
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