To fix Steam error code E84, right-click the Steam shortcut, open Properties, and add -noreactlogin to the Target field.
To fix E84 efficiently, it helps to know what's breaking.
The error sits at the handshake between the Steam client and Valve's authentication servers.
If the seven fixes above haven't landed and you'd rather not reinstall yet, two often-overlooked culprits are worth a minute each.
A parameter fix that works today can stop working after the next client update, because Valve sometimes deprecates old launch flags.
Read the complete breakdown, fixes and what happens next.