networking – why do I see the 504 error for

I'm trying to access the UK's official train website, but I still see a 504 error – Gateway Timeout on my laptop connected to the wifi, and then to the fiber. The site is up, as confirmed when I access the page via the telephone network of my cell phone.
Therefore, I guess the mistake comes from my side and not from them. I have tried

  • 3 different browsers
  • disable the firewall on my laptop (littlesnitch)
  • restart my router
  • disable my pihole advertising blocker

What could cause this error and how can I solve this problem?