Virtualmin post-installation problem

I'm trying to run Virtualmin on my CentOS 5 VPS. It's a clean install and I've followed the instructions as per the instructions here - http://www.webmin.com/vinstall.html

Prior to installation, I checked that I had a FQDN set, using hostname -f, which returned the following:

server1.DOMAIN.org.uk (DOMAIN used in place of the actual name)

Following the installation, I get this message when I try to access the server IP on port 10000:

Error - Bad Request

This web server is running in SSL mode. Try the URL https://server1.DOMAIN.org.uk:10000/ instead.

I try the link mentioned, but that just says the page is unavailable.

I've tried this several times now and all result in the same error. What should I start to look at to resolve the problem?