I was having the same issue all the day too. Thought that my Yahoo Messenger was corrupted so tried Meebo, but also could not login .. after like 30 minutes of countless numbers of tries ... I was able to login, tried to open my e-mails ... got the 999 error on yahoo server. Guess that some of their servers were down for maintenance. Remember getting that error at the beginning of the lunch of the new Ajax interface of yahoo messenger.

You can try Yahoo Web Messenger ... it works fine with me, the funny thing is after using that I tried connecting using my normal YM and it works fine :)

Gotta bear with Yahoo issues, nothing wrong from your side. It is their problem.