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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/maps1/23375/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-17dce822-c6b9-497d-a5a1-18dbfb289dd8",
      "cw-1d494dee-2c3c-4217-bf77-3a25a2bc3407"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-eb7a831e-0ad7-4bb7-9f08-9e7c1c70e6c4",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/23375/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-1d494dee-2c3c-4217-bf77-3a25a2bc3407"
  }
}