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/maps1/1506/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-a5f0460f-3158-4847-b02f-a37826674588",
      "cw-6c3778eb-1467-4d88-951f-d4b86455ab6a"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-7f6f7171-d6bf-434c-8f50-d662c61dc156",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/1506/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-6c3778eb-1467-4d88-951f-d4b86455ab6a"
  }
}