Yahoo recently made security updates requiring you to use app passwords (as opposed to your own user-created password) when connecting to your Yahoo Mail account through a 3rd party email client. I'll post the link here as well: https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html. It doesn't matter what your own password is -- if you want to use an email client, you probably have to use an app password that's generated by Yahoo.
You might find it interesting that I have 3 yahoo accounts. Interestingly I was able to sync one of them. The remaining two have the same problems. One stopped on Feb 10. The other on Feb 15. I didn't have to do any generated weird password for one of the yahoo accounts. Again, I'm not the only one with the problem according to what I'm seeing. I'm simply giving up for now.