Your World of Warcraft installation is damaged.
Delete the CascRepair file in the Data/data folder.
If your installation was not actually damaged you will be able to log in.
Stuck on Connecting / Disconnected from Server
Make sure the Config.wtf inside the WTF folder has the line: SET portal "play.epsilonwow.net"
Make sure your installation has the tc_bundle.txt file found in the tc_bundle.zip file located on the How to Connect page.
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