Network/Web Site Authentication

Hi,

I've got a WSUS server, which has been working fine for nearly a year. For
some reason, in the past month or 2, when I try to manage the WSUS service
from the web console, using the server name (https://wsuserver/WSUSadmin) I
get an authentication error. The authentication box pops up, asking for
username & password, however no matter what credentials I enter, (mine,
domain admin, enterprise admin) it pops up 3 times, fails, and then I get
the WSUS message:

Access denied
Network policy settings prevent you from accessing this Windows Server
Update Services server.
If you believe you have received this message in error, please check with
your system administrator.

However, if I connect using the servers IP address, NOT hostname,
(https://192.168.0.10/WSUSadmin) it works perfectly. I'm faily sure it's not
an IIS setting, as I've setup a test server with WSUS installed, that works
with hostname, and exported the web site to an XML file, then imported it
into the live WSUS server. Also the live & test servers are both in the same
OU, with the same group policy applied, so all the security settings
*should* be the same.

What security setting would cause authentication to a hostname to fail, but
to an IP address to work?

Cheers

Ben