Esumsoft Forums

It seems many users are currently experiencing issues with Yahoo/AOL, and this started late yesterday

The problem is not specific to POP Peeper and affects users of various clients:
https://downdetector.com/status/yahoo-mail/

It does not affect all users -- I personally have 3 active Yahoo/aol accounts and have not had any problems.
edit: It does not affect all accounts -- users have reported that some of their Yahoo accounts are not affected.

Possible work-around: the problem may be specific to Oauth2 logins so you may be able to use app-passwords instead of Oauth2:
https://help.yahoo.com/kb/SLN15241.html?guccounter=1
Once you have an app-password, edit the account in POP Peeper, change the "Oauth2" dropdown to "Password" and enter your app-password. You will need to create a separate app-password for each account and enter that specific password into POP Peeper.

If you try this method, please reply back with your results here.

Yahoo is aware of the issue; this is posted on their help pages:

"We are aware some users are experiencing issues accessing their Yahoo Mail using third-party mail clients. Removing and re-entering your Yahoo Mail password or app password in the email client should solve this. Read our help article for more information and steps. You may also access Yahoo Mail on the web at mail.yahoo.com or using the Yahoo Mail app for iOS or Android."

Yahoo has updated their banner (above). The link they mention is:
https://help.yahoo.com/kb/mail/SLN36847.html

Most POP Peeper users with the recent Yahoo issues have reported that resetting Oauth2 has allowed their accounts to work again.