This has happened before. I have two Yahoo accounts in eMClient. On the main one I get a dialogue box that says ‘authentication failed’ yet the other seems fine. I keep resetting the password to no avail. I am wondering how one address can fail and not the other when they are both Yahoo accounts. In the past these have cured themselves, but this time is a real puzzle.
You could check that you have the exact same settings for both accounts in eM Client. If that does not correct it then the problem probably lies with Yahoo. You could contact their support for more assistance.
Recently, there were situations when Yahoo changed their security policy and automatically changed the account’s security settings. Then users experienced such behaviour as you explained. Please go to the Yahoo account’s security settings (on webmail) and check if the “allow less secure application” option is marked.