I have exactly the same issue on the Sylvanus server. All of my friends can't start the application either. It just hangs in task manager and brings up:
http://legion.fstorm.eu/26365/versions:
World of Warcraft was not able to connect to the network data source. For help fixing this issue, please see: https://battle.net/support/article/7411
even though my internet connection is perfectly fine.