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/661/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-d3531348-68b7-48db-98ee-9f99e958cb89",
      "cw-10f2a9dc-1f31-468e-8c65-046fcbe90334"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-079d78be-992d-4301-b87c-b21a07568900",
    "manifestId": "https://cdm16022.contentdm.oclc.org/iiif/info/mgs/661/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-10f2a9dc-1f31-468e-8c65-046fcbe90334"
  }
}