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/maps1/21979/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-45ac6d00-b8dd-45b8-8b1a-58263e194e73",
      "cw-3ab509bd-0a45-4873-869e-beaeb39f4e9a"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-059aaf2c-0dd3-4616-9cfe-a4d85b44adb8",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/21979/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-3ab509bd-0a45-4873-869e-beaeb39f4e9a"
  }
}