I had this error BLZBNTAGT00001389 on my Mac and it “seems” like I have managed to fix it.
It was every once in a while getting stuck on updating (It would never start) and would give that error message if I tried to click it. Would fix on a reboot.
I used the activity monitor (Task manager I think in windows) and had to manually delete all the battle net stuff which was still somehow lurking there.
Not sure if that will help your issue, but it does, great 