Hello,
I have been having this issue since I reactivated, bought Shadowlands and then Subbed - can Login to WorldofWarcraft site but it doesnt show my character, then on eu battle net site - I get constant “Somethings went wrong” when trying to go to Account Settings.
I login to launcher and it says the error message BLZBNTBNA00000A8E. I did try other browsers, I deleted cache etc - got to login then had a loop of asking me to set up DoB and email etc - then just went back into the loop of “Somethings went wrong”.
I have opened a ticket - but wanted to see if anyone has any ideas.
As a sidenote - I have a US account - which i can login to, play WoW etc etc.
Googling that error shows it’s an issue at the Blizzard end of things to do with a BattleTag not being correctly set with an account. I’m guessing you haven’t played for a while ?
This is detailed here: https://eu.battle.net/support/en/article/65035
Because you can’t get to the Account Settings part of the Blizzard website it may be that you’re unable to resolve this yourself. Some suggestions for you, though:
Try launching the Blizzard Launcher with administrative privileges. This is suggested in the article I linked. I think it’s worth a shot.
Have you tried launching WOW directly from the application rather than through the Blizzard Launcher ? This might get you further into the process - and, try launching it with administrative privileges.
Just in case you’re not sure how to do this, find the application in question and right-click it, and select ‘Run As Administrator’ from the pop up menu.
If none of this helps I think you may have to ask for help. This article details how to raise a ticket when you are unable to log in. https://eu.battle.net/support/en/article/43885
I can only echo the excellent advice given by Deliria here - please feel free to reach out via ticket, if were is indeed a need to get a battletag set up. We should be able to get that done rather quickly for you. 
Just to reply - thanks for the advice, it is all fixed now.