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/maps1/15685/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-3da7fd49-344a-43f2-9647-39dca4cd8cdf",
      "cw-1a7c4f36-1d5b-4a1c-9a2e-abe340ebdf0f"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-c90f1c80-033b-458a-8b16-0b9111c2def2",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/15685/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-1a7c4f36-1d5b-4a1c-9a2e-abe340ebdf0f"
  }
}