This is to fix Error 131, Files corrupt when trying to log in.
Go into your World of Warcraft folder, find the folder "Data". There is a file called patch.MPQ. If it is about ~700-800mb, you gotta change it. To do so:
Delete current patch.MPQ.
Run repair tool and click "yes" when promted to revert to older version.
Install full 2.4 patch. (1,1gb)
If you need to find a place to DL full 2.4 patch, look here:
Patch mirrors - WoWWiki - Your guide to the World of Warcraft
Good luck, this helped me, after searching alot.