Yahoo's IMAP server is currently reporting the following error:
[AUTHENTICATIONFAILED] AUTHENTICATE Invalid credentials
This is a server-side error and it affects all IMAP clients. They are aware of the issue:
https://forums.yahoo.net/t5/Known-issue ... d-p/456453
In the meantime, you can consider switching POP Peeper to access your account using POP3 or WebMail (edit the account and change the 'server type' to 'POP3' or 'Yahoo') but be aware that POP Peeper will need to download all existing email in your inbox whenever you change the protocol. If you had been using OAuth2, be sure to enter your password when accessing with POP3 or WebMail.
You can track the issue here:
viewtopic.php?f=3&t=7066
Yahoo IMAP issues
Re: Yahoo IMAP issues
Yahoo has officially fixed the problem.