I've just come back to do a "Phase II" of an old EPiServer 4.61 project and was having issue's with none of the child pages working, or even the CSS being loaded in the CMS area. I decided to investigate the style sheet error in the admin area - it was looking for /util/styles/system.css but if I looked in the file system such a file didn't exist... hmm! Then I realised it must be a virtual path and remembered that pre version 5 EPiServer used a custom error page to return virtual files and to do its friendly URL re-writing. So I looked in IIS and bingo there was the problem.... IIS was using the default 404 error page, once it was changed to /Util/Notfound.aspx everything sprung into life...

Tip from Steve: If you Open EPiManager and click on the offending site you'll get a warning message saying that IIS's configuration is incorrect - do you want to fix the 404 error page value! If only I'd thought to look there earlier!!