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/maps1/20458/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-95adcdcb-0b32-4a7e-9612-0f08c726bcf3",
      "cw-8e10edbd-a88b-4c86-a57f-3a370521cfbd"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-22147a8d-17b2-43a6-bbe1-37ed749557a5",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/20458/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-8e10edbd-a88b-4c86-a57f-3a370521cfbd"
  }
}