Mirador viewer

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

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "/pid/NHMW-AFW-ADMIN-IA-1886-0275/manifest.json?s=1",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-9b24ba0f-d15e-4cda-ac99-16c598972ecc",
      "cw-c0a73536-9657-480b-bc8f-dcc9dcfff5e8"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-d84dc785-250d-4d0d-b934-c8842c306d4d",
    "manifestId": "/pid/NHMW-AFW-ADMIN-IA-1886-0275/manifest.json?s=1",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": true,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-c0a73536-9657-480b-bc8f-dcc9dcfff5e8",
    "thumbnailNavigationPosition": "on",
    "loadedManifest": "/pid/NHMW-AFW-ADMIN-IA-1886-0275/manifest.json?s=1",
    "canvasIndex": 1,
    "availableViews": [
      "ThumbnailsView",
      "ImageView"
    ]
  }
}