Mirador - viewer

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

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "/api/digital_items/139846/2/manifest.json/",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-70b865c3-cb42-4304-a099-b8be14080194",
      "cw-12ae3f2e-fdeb-4d5b-ae72-5eb2509d401a"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-a8d9588b-a58d-47f9-a857-60b7df55e02a",
    "manifestId": "/api/digital_items/139846/2/manifest.json/",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "annotations",
    "thumbnailNavigationId": "cw-12ae3f2e-fdeb-4d5b-ae72-5eb2509d401a",
    "thumbnailNavigationPosition": "off",
    "loadedManifest": "/api/digital_items/139846/2/manifest.json/",
    "canvasIndex": 0
  }
}