As of May 6th @ 7:05 p.m. (US EST), Yahoo's servers are down. The problem reported by a number of users is being stuck in a login loop.
With IMAP accounts, you may getting Oauth2 errors in PP.
Since this is not a POP Peeper issue, there is nothing to do except wait until Yahoo fixes the problem(s) on their end.
It would appear to be regional -- I am currently (did have an error that recently cleared using IMAP) not having issues with any of my 8 Yahoo accounts (some using any one of IMAP/POP3/Webmail protocols).
Good judgment comes from experience and a lot of that comes from bad judgment. - Will Rogers