Mirador viewer

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/maps1/21531/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
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      "cw-1f9af411-9988-4b63-a4e6-955ee6d835ba"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-c60a9d30-ecd0-4918-a2db-43fac2799aa7",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/21531/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-1f9af411-9988-4b63-a4e6-955ee6d835ba"
  }
}