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/maps1/6791/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-fe485f4c-4de1-4779-b6a6-f9fc8c0df321",
      "cw-3f29425c-c619-41c0-9482-14179bd7ffb8"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-327cf451-69cc-47a3-94c2-a2e6f6e2b650",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6791/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-3f29425c-c619-41c0-9482-14179bd7ffb8"
  }
}