The info at Mickey$oft on "error 404" indicates that it often happens when there is a problem with the "address" you clicked on to get in. An example they give is if there are "spaces" in the address, but any extraneous crud could do it.

So far as I've seen, there is no mudcat address/folder with the "null" name, so that may be part of the problem.

The message says, roughly, that:
you're being given "message 404"
because it can't find a location named "/null"
which is included in the address "c:\webserver\mudcat\htdocs\null"

It's not clear whether this is happening when you first open mudcat, or if it's while you're trying to go to a thread from the discussion menu. If it's happening when you first open mudcat, it's likely that your link has gotten fouled, possibly by that appended "/null."

My suggestion would be that you type the address in your browser address bar - http://www.mudcat.org, or if you usually start at the threads, http://www.mudcat.org/threads.cfm - and see if you get in with a "clean" address. If you do get in, before you navigate around, put the 'cat in your "favorites" link - replacing whatever is there.

You could also try using the "Forum Home" clicky that's at the bottom of this thread page, and if you get there, use the good connection to replace your link - i.e. add to "Favorites" and replace your existing entry.

John