Just in case, to change the password in AquaMail:
- Long press the account
- Account setup
- Enter the new password and press Next, and save at the end
In general, Yahoo is well known for authentication errors, going back to 2007 (if you search the web), all kind of mail apps were/are affected. The symptoms include server error messages about "error OCF12" and "error 999".
I have some info on this in the FAQ, close to the end of the page.