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/858/manifest.json",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-239dc596-c5c5-4df8-96fe-5dda55dc2544",
      "cw-7acc3d43-378f-47f1-9685-6c636a625410"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-c7b10e2c-43b3-4585-81cf-35b4fba55b1b",
    "manifestId": "https://cdm16022.contentdm.oclc.org/iiif/info/mgs/858/manifest.json",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-7acc3d43-378f-47f1-9685-6c636a625410"
  }
}