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/maps1/20028/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-5f21d9c3-94e2-4333-bcd3-ccca15cefbfe",
      "cw-5e5043e0-5694-470e-9eec-dd4862225641"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-d411c16b-f861-4252-a314-2b8d0a4ecde5",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/20028/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-5e5043e0-5694-470e-9eec-dd4862225641"
  }
}