Possibly applicable, even though Netscape has been mentioned as a browser where the problem sometimes occurs:

Microsoft Knowledge Base Article - 827667
"HTTP 404 - File Not Found" Error Message When You Try to Visit Web Pages That Are Opened by JavaScript Functions in Frames or in Windows

SYMPTOMS
After you apply the cumulative security patch for Internet Explorer that is included in Microsoft Security Bulletin MS03-032, you may receive the following error message when you try to visit Web pages that are opened by JavaScript functions in frames or in windows:

         HTTP 404 - File not found

CAUSE
This problem occurs because the cumulative security patch for Internet Explorer that is included in Microsoft Security Bulletin MS03-032 changes the base URLs of JavaScript functions to the direct scripting sites. If the JavaScript functions use relative URLs to open Web pages, this change will cause the URLs of the Web pages to be incorrect and you will receive the error message that is in the "Symptoms" section of this article.

RESOLUTION
A supported hotfix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Only apply it to systems that are experiencing this specific problem. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, Microsoft recommends that you wait for the next Internet Explorer 6 service pack that contains this hotfix.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the hotfix. For a complete list of Microsoft Product Support Services phone numbers and information about support costs, visit the following Microsoft Web site:
http://support.microsoft.com/default.aspx?scid=fh;[LN];CNTACTMS
Note In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question.
Prerequisites
You must apply the cumulative security patch for Internet Explorer that is included in Microsoft Security Bulletin MS03-032 before you install this hotfix. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
822925 MS03-032: August 2003 Cumulative Patch for Internet Explorer


Note that this is a fairly old article, and that the "fix" that caused the problem was the August 2003 update. There has been at least one more IE update that may have fixed this as part of the upgrade. For WinXP and Win2K users, I'd recommend the IE6 upgrade, and then get it current to latest bulletins. (I have IE6 on my Win98 machine, but I use that one so seldom for web stuff that I can't really say whether it's a "recommended" update there.)

Even if you use another browser, Windows has to have current IE to operate those other browsers.

Since few people have the problem, I'd still be inclined to think it's coming from some server in your path to the 'cat, rather from mudcat, but I guess that's something to figure out - - - eventually.

And since those who've been most affected seem to be up to date on everything else, it's just a "shot in the dark" that anyone may need IE updates.

John