Just a hunch, but try this:

Create a simple text file on the IT with your login ID and password clearly spelled out.
Then copy and paste the details into the relevant fields on the Yahoo log-in page. Then hit enter.

My IT has zero problems logging in to Yahoo for mail (auto log-in is perfect) on my n810, so I know it's possible.

The work-around I suggest, was absolutely necessary for me with a couple of sites though, not sure why.