This error usually means that the game folder is corrupted beyond repair, usually from trying to connect to a private server. You should go ahead and run the repair service in your World of Warcraft Folder. If this dosent work, you will probably have to uninstall and then reinstall the complete client. After you uninstall you should run a scan on your computer for anything that involves World of Warcraft files, and delete any that you find (That could be what is corrupting your game).

Here is a secure, safe download if you do have to reinstall the game, there are downloads for both Mac and PC;

However, before you uninstall and reinstall World of Warcraft, I would open the game (using the Blizzard Launcher) to make sure that you are completely patched up to 4.3.4. Then, if you are, run the Repair service to make sure you have the WoW Data Folder and WoW Launcher in your World of Warcraft Folder (The real Blizzard Folder/Launcher for Retail, if you know you replaced them, you should delete the Folder/Launcher you replaced them with, and then put the originals back in your World of Warcraft folder, if you saved them that is. If you didn't save them then you can just try and run the Repair service located in your World of Warcraft folder and that should notice they are missing and replace them).

Once you have your World of Warcraft folder completely back to its original, unaltered state, open it and drag the Data Folder (The real Blizzard one) to your desktop (Or an alternate folder, somewhere where you can find it incase you ever need it back). Once you have done this, re-download the WoWMortal Launcher/Data, unzip the folder, and place the WoWMortal Launcher, and the WoWMortal Data folder into your World of Warcraft Folder. Launch the WoWMortal Launcher and it should work.

Hope this helps! If you see anyone with this problem, send them here. If you need to ask me any questions you can ask on here or send me a Private message, I'm on forums/in-game all day so I'd be happy to help.

All the Best!