Mirador viewer

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

{
  "manifest": {
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON",
    "id": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2F4085%2Fmanifest",
    "isFetching": false
  },
  "window": {
    "collectionIndex": 0,
    "companionAreaOpen": true,
    "companionWindowIds": [
      "cw-1202be5f-521b-4bbb-939e-b89c9b4519cb",
      "cw-19638ba9-4a01-4198-9ab0-384e0c5cad7a"
    ],
    "draggingEnabled": true,
    "highlightAllAnnotations": false,
    "id": "window-b9efb25e-5f51-4ea1-9aa4-de6b62954977",
    "manifestId": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2F4085%2Fmanifest",
    "maximized": false,
    "rangeId": null,
    "rotation": null,
    "selectedAnnotations": {},
    "sideBarOpen": false,
    "sideBarPanel": "info",
    "thumbnailNavigationId": "cw-19638ba9-4a01-4198-9ab0-384e0c5cad7a",
    "loadedManifest": "https%3A%2F%2Ffotografia.cultura.gov.it%2Fpresentation%2F4085%2Fmanifest"
  }
}