BLZBNTAGT00000840 how to fix it
I would love to know if this got any answers as I have the exact same issue atm.
If you are experiencing the BLZBNTAGT00000840 error when trying to launch WoW, try restarting your computer to see if it resolves the issue. You may also want to check the Details section of Task Manager to make sure wowvoiceproxy.exe is closed before you launch the application.
Restarting seems to have worked.
Glad to hear it!
Going to task manager and ending all Blizzard/Wow related things running including the wowvoiceproxy.exe helped the game update for me