What I did was close my wow.exe, open WoW folder and rename the folder named WTF to WTF-OLD. You can rename it to w/e you like. Then you start wow.exe again and viola!
After renaming the folder and u still get the same error message, restart your computer. Thats what worked for my both cmputers.
What you do here is remove your old account UI settings to the Blizzard standards. Because when you start WoW again a new WTF folder will be made.
Yours,
Prox