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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/maps1/28112/manifest.json",
    "isFetching": false
  },
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    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-6882cec3-d2b3-4820-be30-75d4f5c23423",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/28112/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-8a31f0eb-33f2-4875-835d-304d237a1c6d"
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