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/maps1/6588/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-c5a56ebf-153d-4948-95da-ec411be43970",
      "cw-cd3d5460-dd7c-420e-a54b-747f6229c223"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-257b8ac5-bd73-450d-bee2-2dbcbfbf8538",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/6588/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-cd3d5460-dd7c-420e-a54b-747f6229c223"
  }
}