We assume you mean when you "log in" at mudcat or don't log in at mudcat, i.e. a "site login."
You could also mean when you "log in" on your computer or not. You may normally "sign on" when you start up your computer. With recent Win versions you can also operate as "generic user" and may have different security and privacy settings for that user - and no 'cat cookie. A "machine login." While I don't think this is what you're talking about, it would be a lot easier to guess at reasons for your problem if it was.
Most browsers allow a variety of security, privacy, and content settings. The content settings include the famous "porn filter" functions, and usually allow either whole sites or individual pages to be blocked, by site address, and/or can be set to block "based on content." Most of them can be set to block "chat rooms" and/or "discussion" sites.
If you intentionally set up the content filter functions, you would probably use a password; but if they've never been (intentionally) turned on, you often can set them without a password, and it can be pretty easy to mess up the browsing experience by making casual privacy rules. With some browsers it's easy to inadvertently apply a rule, without realizing you've done it, especially if no password has been applied. In IE, as an example, you have to "turn on" the Content Filter in order to look at what can be done there, and it's easy to leave it on when you exit Tools. Even if it appears that no rules were turned on, the "blank" filter can affect your browsing.
If you have "looked at" the "Content Filter" or its equivalent in your browser, it could be the source of the symptoms described.
If anyone else has had access to your computer, they may have inadvertently or deliberately changed content filter use and/or security and privacy settings. The truly malicious could even enter a password and change the settings, if they had "your" or "superuser" authority on your machine. (The password used cannot be broken or discovered by any normal means, so if you ever do use one be sure to "remember it in writing.")
While the Content Filter is supposed to be able to produce the symptoms described, recent Windows and IE updates have applied much more aggressive rules, and a too high Security tab setting could possibly be to blame.
Most of the above assumes you have control of your own machine and connection. If you're on a nework, particularly at a workplace, your SysAdmin may have found someone "wasting time" at the 'cat and just blocked the "Threads" site for everyone. He can do that - and ignorance can be damnable.