With the proliferation of hazards and annoyances on the web, various programs choose many different criteria for what to block. Sometimes its not a single thing, but a combination of things that result in blocking.

One reason for blocking a site might be absence of a "certificate" that identifies the site as at least legitimate enough to have registered with an "authenticator" site. Lots of sites don't bother, and since certificates must be renewed periodically you'll find lots of perfectly good sites that a good program will warn have "invalid" or "unknown" certificates. Many programs only look for certificates if a download of information (saved other than in temp space) is required, so sites may "certify" downloads but not bother with keeping current site certs.

I haven't checked, but I don't believe mudcat presents a cert.(?) (The cat's been on my always allow list for so long I don't remember.)

Many programs are suspicious about sites with popups on their opening page, but usually only the popups are blocked. A good program should tell you when a popup is blocked and provide a way for you to accept it, since on some sites the popup is the content. The utilities that actually do the downloads of things like driver updates on many vendor sites are in this category.

Some programs block all cookies, and/or notifiy you and ask whether you want to accept one. Mudcat actually uses lots of cookies. You need your login cookie to be anything other than "Guest" but you also get (apparently) a separate temporary cookie for each thread you open. The "thread" cookies are how the 'cat knows to change the color of threads in the list that haven't changed since you looked at them.

Programs can filter based on content. The Google toolbar has a "safe search" filter that won't report "objectionable content" sites in search results if you leave it turned on (the default setting).

Since there are lots of threats, and lots of annoyances, that aren't strictly viruses, many AV programs apply complex criteria to "make your browsing safe and happy." You have to learn what yours does, and make adjustments where needed.

John