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/721/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-b9d8e7c9-d8ba-42d0-bcd5-bab4d3322de5",
      "cw-e86caffa-38a7-4fe8-9cc3-c876d8b24067"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-834577a6-6fc1-49ce-a643-e751f3b5ff6d",
    "manifestId": "https://cdm16022.contentdm.oclc.org/iiif/info/mgs/721/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-e86caffa-38a7-4fe8-9cc3-c876d8b24067"
  }
}