Mirador - viewer

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

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "/api/digital_items/3648/2/manifest.json/",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-704266e7-40fd-460a-aaff-9072f0029aec",
      "cw-e1ba6d69-f74e-4162-ace3-19e56d1fd50b"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-2d1e3204-d52c-46c4-b2af-4821d2eb26d9",
    "manifestId": "/api/digital_items/3648/2/manifest.json/",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "annotations",
    "thumbnailNavigationId": "cw-e1ba6d69-f74e-4162-ace3-19e56d1fd50b",
    "thumbnailNavigationPosition": "off",
    "loadedManifest": "/api/digital_items/3648/2/manifest.json/",
    "canvasIndex": 0
  }
}