Problems load WWW page from single server.

Hi.

I have setup a linux server which splits the network into two segments.
The first subnet NAT is performed, and the second simple routing it
performed.

The problem i'm having is that some computers on the network are unable
to access the webserver on the linux server. The strange thing is that
a few days prior they were able to access the www server.

The firewall on the linux server has not changed, and there is not
firewalls installed on the client machines.

The client machines login to a domain controller (SME server) which is
on the external side of the linux server, but that all works fine so I
am unsure domains are related to the problem.

All machines on the network are able to access www servers online, its
just this one linux server.

Has anyone any ideas to what could cause this ?

Thanks.