Mirador viewer

SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https://cdm16022.contentdm.oclc.org/iiif/info/mgs/821/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-c1038cf9-6419-427f-91e4-447fad9e7190",
      "cw-6495d726-358c-434d-8a89-687d59f3b14b"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-de254049-8094-4c63-94f9-5e946c918e04",
    "manifestId": "https://cdm16022.contentdm.oclc.org/iiif/info/mgs/821/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-6495d726-358c-434d-8a89-687d59f3b14b"
  }
}