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/maps1/28573/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-451b83e4-185c-49e4-87e8-9775ead70b14",
      "cw-c9a1d38a-b267-469a-a5ed-56493d8c7bf4"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-114fb8b2-0367-4b22-85e7-c145382dea61",
    "manifestId": "https://digital.libraries.psu.edu/iiif/info/maps1/28573/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-c9a1d38a-b267-469a-a5ed-56493d8c7bf4"
  }
}