Security certificate not trusted

One of my clients is having this on going issue with customers calling up saying that their web browser is saying “The site’s security certificate is not trusted”.

I’ve followed the instructions to install the intermediate certificates (even the updated ones sent earlier this year). There are three load balanced web servers with the certificate installed. I’ve read various posts on here.

The frustrating thing is that I cannot reproduce this error. The latest customer was using Google Chrome, but every browser I test, I get the green bar and all is fine.
Conversion rates are healthy, so it can only be a few users getting this message, but without being able to reproduce this error it’s kind of hard to tell.

The site in question is

Anyone come across this problem?

Are you using IIS at all? It would appear that you are as the certificate chain output would indicate as so. This site is not using the full chain, thus some clients may have a problem with the half chain that is being presented.

Yes I’m using IIS6. How do I rectify this chain issue then?