Sometimes you might find that you can not login your yahoo messenger with correct account name and password. You tried lots of times but still failed to login.
I tried a way and found it worked well:
Please try changing your connection settings and try to login, you should get error message since you set wrong connection setting. Then please change the setting back after a few minutes or hours.
Where to change setting?
go to messenger > preferences > connection, then change the setting to any other one and try to login , later change it back and try to login again.