Description :

On the logging screen or while you play, a window appears saying that a Lua error was encoutered.

Solution :

1. Try do disable all your add-ons.
2. Empty your cache :
   1. Close the game.
   2. Go in your game folder.
   3. Delete the "Cache" folder.
   4. Launch the game.
3. Reset your interface :
   1. Close the game.
   2. Go in your game folder.
   3. Move the interface folder where you want (out of the game folder).
   4. Launch the game.
4. If you tried all the solutions above and the error still appears, here is something that will kill it :
   1. In the in-game chat, write down the following command and press enter : /console scriptErrors 0