After several attempts with Yahoo and Verizon support, my new-best-friend, Anthony, (Verizon) has solved the issue that has recently been plaguing many of us. On the Verizon side, I had to change my password to something never used before and ONLY with letters/numbers (no special characters). Once I did that, I was able to login to mail.yahoo.com!
I don't know if it was only the special characters in my PW that were throwing things off, but maybe Yahoo either doesn't allow them or is having some issue with them at the moment. Give it a try!