I got that problem as well several times during the day and several things worked to get back on the game again. You should try this before start removing addons, or re-installing etc.
When starting the battle.net and then world of Warcraft, do not click on other stuff until you are logged in proper!
- Run the battle.net as Administrator.
If it doesn’t work
- Restart your comp and then battle.net. Then start worldofwarcraft
This worked for me, but may not work for you. But, I really hope it does so we can enjoy this awesome game!
- FAST Tactics on Azshara
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