NC24: /core/preview: "Host 192.168.0.23 was not connected to because it violates local access rules"

NC 24.0.2 + Ubuntu 20.04 + PHP 7.4

Fresh installed NC and fresh created user.

Thumbnails were created but a warning while watching freshly generated thumbnails in the NC files browser occurs

{
  "reqId": "7I0j9jZoGSWkTlYeiDHg",
  "level": 2,
  "time": "June 23, 2022 15:42:44",
  "remoteAddr": "192.168.0.2",
  "user": "test",
  "app": "no app in context",
  "method": "GET",
  "url": "/core/preview?fileId=449195&c=7aa076f0da64bbfe89c75c707d9ac6fc&x=250&y=250&forceIcon=0&a=0",
  "message": "Host 192.168.0.23 was not connected to because it violates local access rules",
  "userAgent": "Mozilla/5.0 (X11; Linux x86_64; rv:101.0) Gecko/20100101 Firefox/101.0",
  "version": "24.0.2.1"
}

What does this mean?

And how to find out the file which cause the error?

In that case: fileId=449195

Or in short: how to debug it?

Thank you!