Hello,
Due to Google restrictions we are forced to revert the way Aqua Mail handles OAUTH2 for Hotmail, Yahoo and Yandex. We will implement the new modernized OAUTH2 for them in the near future but it will take some time.
Now if you try to add Yahoo account, you should go to your web mail under Account Security and turn on the second slide box on the bottom right.
If you still have problems, please contact us at support@aqua-mail.com.
Thank you!
Nadia