go into your wow directory and right click WoW.exe.
go to properties. go to the version tab and you should see
File Version: 2.1.0.6692 (something similar)
take note of the last 4 digits.
then open your logonserver.conf and look for this
Code:
# Accepted Build Range Setup
#
# These two directives set up which clients will be
# allowed to authenticate with the realm list.
#
# Set these to the same builds that the server was
# compiled for.
#
# As of the last update, version 2.1.0 was build 6692.
#
MinClientBuild = 6692
MaxClientBuild = 6692
make sure the numbers match up.