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/AOL Oauth2 issues
Re: Yahoo/AOL Oauth2 issues
Yahoo is aware of the issue; this is posted on their help pages:
Re: Yahoo/AOL Oauth2 issues
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.