I just tested it - it is possible to log in (after login, I see the "you have xx messages" at the top of the page), but whatever I do after that, the login is lost.

My guess: one of the cookies uses .mudcat.org as domain (the other two use www.mudcat.org). Probably w3m does not recognize the .mudcat.org cookie should be sent with the requests, thus the mudcat does not recognize the login any longer.

I think the .mudcat.org cookie comes from the time when there were lots of servers at mudcat.org ...

Maybe Jeff can change the cookie to have www.mudcat.org as domain.