Hi, there’s solution:
Link your account to any other platform google or whatever you like. Enter the game, and simply click register. Then choose registration with whatever you’re linked in. Game gonna crash, because you’re trying to register already registered account. After reopening it you will be logged in to earlier connected account.
If you need more help, dm me on discord
Crue4u
Same problem here, was able to play last week and now impossible to connect to my account on phone…
Hi everyone! I don’t know if this solution will work for you but it did for me! I was getting the exact problem as OP.
Go into apps in your phone’s settings and delete it’s data(mine had 6gb data before deleting, down to 503mb after). When you open again, log in to your account there on the first screen, otherwise it will create a new account for you and that is the existing account it confuses with yours. I noticed every time I tried to log on previously, in my account manager, it created one every time I logged in after deleting it, so I had “generic blizz account” and “original old account that I’ve used before” on my device clashing with each other to log in.
I have thr same problem. Marge my battlenet account with google doesnt worki.
I had the same problem, here is what helped me:
When I open hs on my phone, it automatically logs in to a newly created account (that’s the reason I can’t log in to my old account).
Open settings, skip apprentice (you may have to take the rewards from journal before), you skip it, then you open settings again, and you change region. Once the new and the old account is on different regions you’ll be able to log in to your old account.
Thank you
This help my 
Thankfully I had not played long when I ran into this. Clearing cache and data and thus purging the Guest progress on the app let me log in.