Your system information

• Steam client version (build number or date): steam 1.0.0.82-2 (https://gitlab.archlinux.org/archlinux/packaging/packages/steam)
• Distribution (e.g. Ubuntu): Arch Linux
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

Describe what you expected should happen and what did happen. Please link any large code pastes as a Github Gist

Steps for reproducing this issue:

1. have iptables with outgoing and incoming packets for loopback to deny
2. start steam
3. steam crashes with "Steam encountered an unexpected error during startup (0x3000)".

nftables config

table inet filter {
  chain input {
    type filter hook input priority 0
    policy drop

    ct state invalid drop comment "early drop of invalid connections"
    ct state {established, related} accept comment "allow tracked connections"

    counter
  }

  chain forward {
    type filter hook forward priority 0
    policy drop

    counter
  }

  chain output {
    type filter hook output priority 0
    policy drop

    tcp dport {80, 443} accept comment "Allow 80 and 443"

    counter
  }
}

The issue is resolved if you allow outgoing and incoming loopback, but that shouldn't be necessary.

This might be related - https://github.com/ValveSoftware/steam-for-linux/issues/5082