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/763/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-4ca8ca38-5438-416f-96bb-4c0126c04dec",
      "cw-9a30a0be-6f90-467e-834e-a89e10a503e0"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-e5b7596c-703a-4dbb-ba77-d97cc8779bb3",
    "manifestId": "https://cdm16022.contentdm.oclc.org/iiif/info/mgs/763/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-9a30a0be-6f90-467e-834e-a89e10a503e0"
  }
}