Many people are getting errors, before you post yours, read this thread.
In patch 2.4.3 the following things occurred (that a programmer would notice)
1.) Opcodes updated - this is not an issue and most emulators have this done already
2.) Encryption - Blizzard decided to do something new, they haven't done before, and encrypt this patch, thus, causing you have to have connection issues. This IS the main issue.
Give ArcEmu and MaNGOS a few days to decrypt this and then you should be good to go, I'll keep you updated as it is needed.
-SectorSeven