This is fixed in the version v3.22 of Thruk. I quote the changelog:

- Apache:
  - add UnsafeAllow3F for Ubuntu packages

You hit this bug https://github.com/sni/thruk/issues/1433 after an apache update.

As a workaround, you can add the flag UnsafeAllow3F manually in /usr/share/thruk/thruk_cookie_auth.include as well.