I have found a solution.
I rummaged around and found my old WOTLK physical copy. Put the CD in, and started Battle.net. It works perfectly now and even after taking out the CD the error is gone for good, I am finally able to play after a month, and the nightmare is over.
If anyone has an old physical CD they can put in and you have this issue, give it a try and see what happens.
I have been away from WoW for a few months, and this problem began right as I was leaving. Just renewed my sub, and this problem was still happening, months later and after patches. I just tried this suggestion; loaded my old WotLK disc and boom: fixed. Thank you for this suggestion, Vyre!
I have experienced the same issue for the last few days and have tried every suggestion in the thread besides upgrading from windows 8 to 10 and still no dice. I’ll be asking for a refund at this point. It’s really disappointing that Blizz hasn’t come up with some sort of comprehensive answer even months later. I’m willing to post any details requested by tech support but based on the previous blue posts above I doubt I’ll be getting any.