I had this problem at work once, although not with Mudcat. I'm not an expert, so this is is 50% out of my arse. I don't think anything's wrong with what I'm saying although I may be simplistic in both ideas and vocabulary.

If you can get to the website by using the IP number but not the www address, it's a problem with the domain name registration.

Could be at the website's end - they fail to register it or it lapses or something. Since everybody else is getting in, that's not what happened.

Servers periodically get this registry. If it gets buggered in some way, your ISP will not know where to find 'www.mudcat.org' - the address isn't connected to the IP number, which is what it uses.

The problem with setting a cookie using the IP address is because of the way Mudcat sets and reads cookies, which is also a number/name problem.

My guess is the problem will eventually correct itself when your ISP gets the latest registry. In the meantime, you can tell 'em THEIR SERVER isn't connecting the domain name with the relevant IP number and no one else posting here has this problem so it's GOT to be them.