I've logged into Yahoo mail about 5,000 times since I bought my N800 a few years ago.

Create a new Yahoo account and see if it works. That at least will tell you something.

Did you ever have Yahoo set up on your N800 so that it automatically logged in? Maybe it's automatically entering the wrong pw.

It's easy to mess up entering the password on a tablet, because the field is small and it's hard to tell whether you have accidentally repeated a character

Tell Yahoo you forgot your password and see what it tells you it is.

It works with no problem for me, and always has. It works on my N800 and on my N810, which I just got.

I gather it worked for awhile. You could always reflash, just for fun.