Error code

Steam VAC Was Unable to Verify Your Game Session

STEAMVAC Was Unable to Verify Your Game Session
Game/Platform Error· An error code shown by a game or gaming platform/launcher.

What it means

A multiplayer game disconnects with "VAC was unable to verify the game session" — not a VAC ban..

How to fix it

  1. Restart Steam as administrator.
  2. Verify integrity of game files.
  3. Close VPNs and background overlay applications.
  4. Restart the Steam Client Service (services.msc > SteamService > Restart).
  5. Flush DNS.
  6. Update Windows and game files.
  7. Temporarily disable antivirus.

Not affiliated with Steam. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.